diff --git a/Tests/test_color_lut.py b/Tests/test_color_lut.py
index 085f07fcc38..196b1943a39 100644
--- a/Tests/test_color_lut.py
+++ b/Tests/test_color_lut.py
@@ -140,7 +140,7 @@ def test_correct_args(
     def test_wrong_mode(
         self, image_mode: str, lut_mode: str, table_channels: int, table_size: int
     ) -> None:
-        with pytest.raises(ValueError, match="wrong mode"):
+        with pytest.raises(ValueError, match="bands"):
             im = Image.new(image_mode, (10, 10), 0)
             im.im.color_lut_3d(
                 lut_mode,
diff --git a/Tests/test_image_convert.py b/Tests/test_image_convert.py
index f596f009025..f72af583ade 100644
--- a/Tests/test_image_convert.py
+++ b/Tests/test_image_convert.py
@@ -46,7 +46,7 @@ def convert(im: Image.Image, mode: str) -> None:
 
 def test_unsupported_conversion() -> None:
     im = hopper()
-    with pytest.raises(ValueError, match="image has wrong mode"):
+    with pytest.raises(ValueError):
         im.convert("INVALID")
 
 
@@ -318,7 +318,7 @@ def test_matrix_wrong_mode() -> None:
     im = hopper("L")
 
     # Act / Assert
-    with pytest.raises(ValueError, match="image has wrong mode"):
+    with pytest.raises(ValueError, match="image must have exactly 3 bands"):
         im.convert(mode="L", matrix=rgb2xyz_matrix)
 
 
diff --git a/Tests/test_image_paste.py b/Tests/test_image_paste.py
index 976b1b2b681..53d63e53c24 100644
--- a/Tests/test_image_paste.py
+++ b/Tests/test_image_paste.py
@@ -370,7 +370,9 @@ def test_overflow(self, box: tuple[int, int, int, int]) -> None:
         im = Image.new("1", (1, 1))
         im.paste(1, box)
 
-        with pytest.raises(ValueError, match="images do not match"):
+        with pytest.raises(
+            ValueError, match="image dimensions and pixel size must match"
+        ):
             im.paste(im.copy(), box)
 
     def test_incorrect_abbreviated_form(self) -> None:
diff --git a/Tests/test_image_putalpha.py b/Tests/test_image_putalpha.py
index 404e3433e57..917e9dd5aad 100644
--- a/Tests/test_image_putalpha.py
+++ b/Tests/test_image_putalpha.py
@@ -41,7 +41,7 @@ def test_promote() -> None:
     assert im.mode == "RGBA"
     assert im.getpixel((0, 0)) == (1, 2, 3, 4)
 
-    with pytest.raises(ValueError, match="image has wrong mode"):
+    with pytest.raises(ValueError, match="RGB"):
         im.im.setalpha()
 
 
diff --git a/Tests/test_imagewin.py b/Tests/test_imagewin.py
index 18f7527a045..73a502e27a9 100644
--- a/Tests/test_imagewin.py
+++ b/Tests/test_imagewin.py
@@ -101,7 +101,7 @@ def test_dib_paste_bbox(self) -> None:
         # Assert
         assert dib.size == (128, 128)
 
-        with pytest.raises(ValueError, match="images do not match"):
+        with pytest.raises(ValueError, match="box must match image size"):
             dib.paste(im, (0, 0, 1, 1))
 
     def test_dib_frombytes_tobytes_roundtrip(self) -> None:
diff --git a/src/_imaging.c b/src/_imaging.c
index 9303dcd34fb..a23aa368b76 100644
--- a/src/_imaging.c
+++ b/src/_imaging.c
@@ -371,14 +371,31 @@ ImagingError_MemoryError(void) {
 }
 
 void *
-ImagingError_Mismatch(void) {
-    PyErr_SetString(PyExc_ValueError, "images do not match");
+ImagingError_Mismatch(const char *message) {
+    PyErr_SetString(
+        PyExc_ValueError, (message) ? (char *)message : "images do not match"
+    );
     return NULL;
 }
 
 void *
-ImagingError_ModeError(void) {
-    PyErr_SetString(PyExc_ValueError, "image has wrong mode");
+ImagingError_ModeError(const char *message) {
+    PyErr_SetString(
+        PyExc_ValueError, (message) ? (char *)message : "image has wrong mode"
+    );
+    return NULL;
+}
+
+// Derives from both NotImplementedError and ValueError
+// (as the functions above raise ValueErrors).
+static PyObject *ImagingNotSupportedError = NULL;
+
+void *
+ImagingError_NotSupportedError(const char *message) {
+    PyErr_SetString(
+        ImagingNotSupportedError ? ImagingNotSupportedError : PyExc_NotImplementedError,
+        (message) ? (char *)message : "operation not supported"
+    );
     return NULL;
 }
 
@@ -2077,7 +2094,7 @@ im_setalpha(ImagingObject *self, PyObject *args) {
     /* attempt to modify the mode of an image in place */
     Imaging im = self->image;
     if (im->mode != IMAGING_MODE_RGB && im->mode != IMAGING_MODE_RGBX) {
-        return ImagingError_ModeError();
+        return ImagingError_ModeError("only RGB/RGBX modes supported");
     }
     im->mode = IMAGING_MODE_RGBA;
     im->bands = 4;
@@ -4319,6 +4336,24 @@ setup_module(PyObject *m) {
         return -1;
     }
 
+    if (ImagingNotSupportedError == NULL) {
+        // NotSupportedError derives from both NotImplementedError and ValueError,
+        // for compatibility with `except ValueError`.
+        PyObject *bases = PyTuple_Pack(2, PyExc_NotImplementedError, PyExc_ValueError);
+        if (bases == NULL) {
+            return -1;
+        }
+        ImagingNotSupportedError =
+            PyErr_NewException("PIL._imaging.NotSupportedError", bases, NULL);
+        Py_DECREF(bases);
+        if (ImagingNotSupportedError == NULL) {
+            return -1;
+        }
+    }
+    if (PyModule_AddObjectRef(m, "NotSupportedError", ImagingNotSupportedError) < 0) {
+        return -1;
+    }
+
 #ifdef HAVE_LIBJPEG
     {
         extern const char *ImagingJpegVersion(void);
diff --git a/src/display.c b/src/display.c
index a950cffc127..cc1d3e2e24e 100644
--- a/src/display.c
+++ b/src/display.c
@@ -135,12 +135,12 @@ _paste(ImagingDisplayObject *display, PyObject *args) {
     if (xy[2] <= xy[0]) {
         xy[2] = xy[0] + im->xsize;
     } else if (xy[2] - xy[0] != im->xsize) {
-        return ImagingError_Mismatch();
+        return ImagingError_Mismatch("box must match image size");
     }
     if (xy[3] <= xy[1]) {
         xy[3] = xy[1] + im->ysize;
     } else if (xy[3] - xy[1] != im->ysize) {
-        return ImagingError_Mismatch();
+        return ImagingError_Mismatch("box must match image size");
     }
 
     ImagingPasteDIB(display->dib, im, xy);
diff --git a/src/libImaging/AlphaComposite.c b/src/libImaging/AlphaComposite.c
index d1a1e7ad34a..a12f7398e66 100644
--- a/src/libImaging/AlphaComposite.c
+++ b/src/libImaging/AlphaComposite.c
@@ -24,14 +24,16 @@ ImagingAlphaComposite(Imaging imDst, Imaging imSrc) {
     Imaging imOut;
 
     /* Check arguments */
-    if (!imDst || !imSrc ||
-        (imDst->mode != IMAGING_MODE_RGBA && imDst->mode != IMAGING_MODE_LA)) {
-        return ImagingError_ModeError();
+    if (!imDst || !imSrc) {
+        return ImagingError_ValueError("imDst and imSrc must not be NULL");
+    }
+    if (imDst->mode != IMAGING_MODE_RGBA && imDst->mode != IMAGING_MODE_LA) {
+        return ImagingError_ModeError("destination image must have alpha channel");
     }
 
     if (imDst->mode != imSrc->mode || imDst->xsize != imSrc->xsize ||
         imDst->ysize != imSrc->ysize) {
-        return ImagingError_Mismatch();
+        return ImagingError_Mismatch("image modes and dimensions must match");
     }
 
     imOut = ImagingNewDirty(imDst->mode, imDst->xsize, imDst->ysize);
diff --git a/src/libImaging/Bands.c b/src/libImaging/Bands.c
index 410b722bde4..ae4bd88ee13 100644
--- a/src/libImaging/Bands.c
+++ b/src/libImaging/Bands.c
@@ -23,8 +23,12 @@ ImagingGetBand(Imaging imIn, int band) {
     int x, y;
 
     /* Check arguments */
-    if (!imIn || imIn->type != IMAGING_TYPE_UINT8) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+
+    if (imIn->type != IMAGING_TYPE_UINT8) {
+        return (Imaging)ImagingError_NotSupportedError("only 8-bit images supported");
     }
 
     if (band < 0 || band >= imIn->bands) {
@@ -80,8 +84,13 @@ ImagingSplit(Imaging imIn, Imaging bands[4]) {
     int i, j, x, y;
 
     /* Check arguments */
-    if (!imIn || imIn->type != IMAGING_TYPE_UINT8) {
-        (void)ImagingError_ModeError();
+    if (!imIn) {
+        (void)ImagingError_ValueError(NULL);
+        return -1;
+    }
+
+    if (imIn->type != IMAGING_TYPE_UINT8) {
+        (void)ImagingError_NotSupportedError("only 8-bit images supported");
         return -1;
     }
 
@@ -185,8 +194,11 @@ ImagingPutBand(Imaging imOut, Imaging imIn, int band) {
     int x, y;
 
     /* Check arguments */
-    if (!imIn || imIn->bands != 1 || !imOut) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imIn || !imOut) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->bands != 1) {
+        return (Imaging)ImagingError_ModeError("source image must have exactly 1 band");
     }
 
     if (band < 0 || band >= imOut->bands) {
@@ -195,7 +207,7 @@ ImagingPutBand(Imaging imOut, Imaging imIn, int band) {
 
     if (imIn->type != imOut->type || imIn->xsize != imOut->xsize ||
         imIn->ysize != imOut->ysize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch("image types and sizes must match");
     }
 
     /* Shortcuts */
@@ -231,8 +243,16 @@ ImagingFillBand(Imaging imOut, int band, int color) {
     int x, y;
 
     /* Check arguments */
-    if (!imOut || imOut->type != IMAGING_TYPE_UINT8) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+
+    if (!imOut) {
+        return (Imaging)ImagingError_ValueError("only 8-bit images supported");
+    }
+
+    if (imOut->type != IMAGING_TYPE_UINT8) {
+        return (Imaging)ImagingError_NotSupportedError("only 8-bit images supported");
     }
 
     if (band < 0 || band >= imOut->bands) {
@@ -277,11 +297,13 @@ ImagingMerge(const ModeID mode, Imaging bands[4]) {
             break;
         }
         if (bands[i]->bands != 1) {
-            return (Imaging)ImagingError_ModeError();
+            return (Imaging)ImagingError_ModeError(
+                "source image must have exactly 1 band"
+            );
         }
         if (bands[i]->xsize != firstBand->xsize ||
             bands[i]->ysize != firstBand->ysize) {
-            return (Imaging)ImagingError_Mismatch();
+            return (Imaging)ImagingError_Mismatch("image sizes must match");
         }
     }
     bandsCount = i;
diff --git a/src/libImaging/Blend.c b/src/libImaging/Blend.c
index 19662abf044..3f861915978 100644
--- a/src/libImaging/Blend.c
+++ b/src/libImaging/Blend.c
@@ -23,15 +23,20 @@ ImagingBlend(Imaging imIn1, Imaging imIn2, float alpha) {
     int x, y;
 
     /* Check arguments */
-    if (!imIn1 || !imIn2 || imIn1->type != IMAGING_TYPE_UINT8 || imIn1->palette ||
+    if (!imIn1 || !imIn2) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn1->type != IMAGING_TYPE_UINT8 || imIn1->palette ||
         imIn1->mode == IMAGING_MODE_1 || imIn2->palette ||
         imIn2->mode == IMAGING_MODE_1) {
-        return ImagingError_ModeError();
+        return ImagingError_ModeError(
+            "images must have 8-bit data and must not use mode P or 1"
+        );
     }
 
     if (imIn1->type != imIn2->type || imIn1->bands != imIn2->bands ||
         imIn1->xsize != imIn2->xsize || imIn1->ysize != imIn2->ysize) {
-        return ImagingError_Mismatch();
+        return ImagingError_Mismatch("image types, band count and sizes must match");
     }
 
     /* Shortcuts */
diff --git a/src/libImaging/BoxBlur.c b/src/libImaging/BoxBlur.c
index b1a8fd04a19..5830e9518f6 100644
--- a/src/libImaging/BoxBlur.c
+++ b/src/libImaging/BoxBlur.c
@@ -253,18 +253,20 @@ ImagingBoxBlur(Imaging imOut, Imaging imIn, float xradius, float yradius, int n)
     if (imIn->mode != imOut->mode || imIn->type != imOut->type ||
         imIn->bands != imOut->bands || imIn->xsize != imOut->xsize ||
         imIn->ysize != imOut->ysize) {
-        return ImagingError_Mismatch();
+        return ImagingError_Mismatch("image mode, type, bands and size must match");
     }
 
     if (imIn->type != IMAGING_TYPE_UINT8) {
-        return ImagingError_ModeError();
+        return ImagingError_NotSupportedError("non-uint8 images not supported");
     }
 
     if (imIn->mode != IMAGING_MODE_RGB && imIn->mode != IMAGING_MODE_RGBA &&
         imIn->mode != IMAGING_MODE_RGBa && imIn->mode != IMAGING_MODE_RGBX &&
         imIn->mode != IMAGING_MODE_CMYK && imIn->mode != IMAGING_MODE_L &&
         imIn->mode != IMAGING_MODE_LA && imIn->mode != IMAGING_MODE_La) {
-        return ImagingError_ModeError();
+        return ImagingError_NotSupportedError(
+            "BoxBlur operation not supported in this mode"
+        );
     }
 
     /* Apply blur in one dimension.
diff --git a/src/libImaging/Chops.c b/src/libImaging/Chops.c
index 557c2f52f10..537fc888be1 100644
--- a/src/libImaging/Chops.c
+++ b/src/libImaging/Chops.c
@@ -68,14 +68,19 @@
 static Imaging
 create(Imaging im1, Imaging im2, const ModeID mode) {
     int xsize, ysize;
+    if (!im1 || !im2) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
 
-    if (!im1 || !im2 || im1->type != IMAGING_TYPE_UINT8 ||
-        (mode != IMAGING_MODE_UNKNOWN &&
-         (im1->mode != IMAGING_MODE_1 || im2->mode != IMAGING_MODE_1))) {
-        return (Imaging)ImagingError_ModeError();
+    if (im1->type != IMAGING_TYPE_UINT8) {
+        return (Imaging)ImagingError_ModeError("images must have 8-bit data");
+    }
+    if (mode != IMAGING_MODE_UNKNOWN &&
+        (im1->mode != IMAGING_MODE_1 || im2->mode != IMAGING_MODE_1)) {
+        return (Imaging)ImagingError_ModeError("both images must have mode 1");
     }
     if (im1->type != im2->type || im1->bands != im2->bands) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch("image types and band count must match");
     }
 
     xsize = (im1->xsize < im2->xsize) ? im1->xsize : im2->xsize;
diff --git a/src/libImaging/ColorLUT.c b/src/libImaging/ColorLUT.c
index 5559de689b2..fc07d3d53ab 100644
--- a/src/libImaging/ColorLUT.c
+++ b/src/libImaging/ColorLUT.c
@@ -87,14 +87,23 @@ ImagingColorLUT3D_linear(
         return NULL;
     }
 
-    if (imIn->type != IMAGING_TYPE_UINT8 || imOut->type != IMAGING_TYPE_UINT8 ||
-        imIn->bands < 3 || imOut->bands < table_channels) {
-        return (Imaging)ImagingError_ModeError();
+    if (imIn->type != IMAGING_TYPE_UINT8 || imOut->type != IMAGING_TYPE_UINT8) {
+        return (Imaging)ImagingError_NotSupportedError(
+            "only 8-bit image modes are supported"
+        );
+    }
+    if (imIn->bands < 3) {
+        return (Imaging)ImagingError_ModeError("input image needs at least 3 bands");
+    }
+    if (imOut->bands < table_channels) {
+        return (Imaging)ImagingError_ValueError(
+            "output image needs at least as many bands as the table"
+        );
     }
-
-    /* In case we have one extra band in imOut and don't have in imIn.*/
     if (imOut->bands > table_channels && imOut->bands > imIn->bands) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ModeError(
+            "output image has more bands than input image and table"
+        );
     }
 
     ImagingSectionEnter(&cookie);
diff --git a/src/libImaging/Convert.c b/src/libImaging/Convert.c
index 4a35a9a1fb1..acad10167cf 100644
--- a/src/libImaging/Convert.c
+++ b/src/libImaging/Convert.c
@@ -1608,13 +1608,13 @@ ImagingConvert(
     ImagingShuffler convert;
 
     if (!imIn) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ValueError(NULL);
     }
 
     if (mode == IMAGING_MODE_UNKNOWN) {
         /* Map palette image to full depth */
         if (!imIn->palette) {
-            return (Imaging)ImagingError_ModeError();
+            return (Imaging)ImagingError_ModeError("image has no palette");
         }
         mode = imIn->palette->mode;
     } else {
@@ -1683,7 +1683,7 @@ ImagingConvertTransparent(Imaging imIn, const ModeID mode, int r, int g, int b)
     int y;
 
     if (!imIn) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ValueError(NULL);
     }
 
     if (imIn->mode == IMAGING_MODE_RGB &&
diff --git a/src/libImaging/Crop.c b/src/libImaging/Crop.c
index 2425b4cd589..7379671fe86 100644
--- a/src/libImaging/Crop.c
+++ b/src/libImaging/Crop.c
@@ -25,7 +25,7 @@ ImagingCrop(Imaging imIn, int sx0, int sy0, int sx1, int sy1) {
     INT32 zero = 0;
 
     if (!imIn) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ValueError(NULL);
     }
 
     xsize = sx1 - sx0;
diff --git a/src/libImaging/Dib.c b/src/libImaging/Dib.c
index 2afe71d4ac9..599f886ba30 100644
--- a/src/libImaging/Dib.c
+++ b/src/libImaging/Dib.c
@@ -59,7 +59,9 @@ ImagingNewDIB(const ModeID mode, int xsize, int ysize) {
 
     /* Check mode */
     if (mode != IMAGING_MODE_1 && mode != IMAGING_MODE_L && mode != IMAGING_MODE_RGB) {
-        return (ImagingDIB)ImagingError_ModeError();
+        return (ImagingDIB)ImagingError_NotSupportedError(
+            "only modes I/L/RGB supported"
+        );
     }
 
     const int pixelsize = mode == IMAGING_MODE_RGB ? 3 : 1;
diff --git a/src/libImaging/Effects.c b/src/libImaging/Effects.c
index c05c5764e44..cee7aeeab18 100644
--- a/src/libImaging/Effects.c
+++ b/src/libImaging/Effects.c
@@ -33,7 +33,9 @@ ImagingEffectMandelbrot(int xsize, int ysize, double extent[4], int quality) {
     width = extent[2] - extent[0];
     height = extent[3] - extent[1];
     if (width < 0.0 || height < 0.0 || quality < 2) {
-        return (Imaging)ImagingError_ValueError(NULL);
+        return (Imaging)ImagingError_ValueError(
+            "width and height must be >=0 and quality >= 2"
+        );
     }
 
     im = ImagingNewDirty(IMAGING_MODE_L, xsize, ysize);
diff --git a/src/libImaging/Fill.c b/src/libImaging/Fill.c
index 6f730921358..3efd74852e4 100644
--- a/src/libImaging/Fill.c
+++ b/src/libImaging/Fill.c
@@ -78,7 +78,9 @@ ImagingFillLinearGradient(const ModeID mode) {
 
     if (mode != IMAGING_MODE_1 && mode != IMAGING_MODE_F && mode != IMAGING_MODE_I &&
         mode != IMAGING_MODE_L && mode != IMAGING_MODE_P) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_NotSupportedError(
+            "only modes 1/F/I/L/P supported"
+        );
     }
 
     im = ImagingNewDirty(mode, 256, 256);
@@ -117,7 +119,9 @@ ImagingFillRadialGradient(const ModeID mode) {
 
     if (mode != IMAGING_MODE_1 && mode != IMAGING_MODE_F && mode != IMAGING_MODE_I &&
         mode != IMAGING_MODE_L && mode != IMAGING_MODE_P) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_NotSupportedError(
+            "only modes 1/F/I/L/P supported"
+        );
     }
 
     im = ImagingNewDirty(mode, 256, 256);
diff --git a/src/libImaging/Filter.c b/src/libImaging/Filter.c
index 3492a871f21..3cbd1ca0090 100644
--- a/src/libImaging/Filter.c
+++ b/src/libImaging/Filter.c
@@ -487,7 +487,9 @@ ImagingFilter(Imaging im, int xsize, int ysize, const FLOAT32 *kernel, FLOAT32 o
     ImagingSectionCookie cookie;
 
     if (im->type == IMAGING_TYPE_FLOAT32) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ModeError(
+            "32-bit floating point image types not supported"
+        );
     }
 
     if (im->xsize < xsize || im->ysize < ysize) {
diff --git a/src/libImaging/Geometry.c b/src/libImaging/Geometry.c
index 1bf92e26efa..85571baf111 100644
--- a/src/libImaging/Geometry.c
+++ b/src/libImaging/Geometry.c
@@ -19,11 +19,14 @@ ImagingFlipLeftRight(Imaging imOut, Imaging imIn) {
     ImagingSectionCookie cookie;
     int x, y, xr;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
     if (imIn->xsize != imOut->xsize || imIn->ysize != imOut->ysize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch("images must have the same size");
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -62,11 +65,14 @@ ImagingFlipTopBottom(Imaging imOut, Imaging imIn) {
     ImagingSectionCookie cookie;
     int y, yr;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
     if (imIn->xsize != imOut->xsize || imIn->ysize != imOut->ysize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch("images must have the same size");
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -89,11 +95,16 @@ ImagingRotate90(Imaging imOut, Imaging imIn) {
     int x, y, xx, yy, xr, xxsize, yysize;
     int xxx, yyy, xxxsize, yyysize;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
     if (imIn->xsize != imOut->ysize || imIn->ysize != imOut->xsize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch(
+            "output size must be the transpose of the input size"
+        );
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -149,11 +160,16 @@ ImagingTranspose(Imaging imOut, Imaging imIn) {
     int x, y, xx, yy, xxsize, yysize;
     int xxx, yyy, xxxsize, yyysize;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
     if (imIn->xsize != imOut->ysize || imIn->ysize != imOut->xsize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch(
+            "output size must be the transpose of the input size"
+        );
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -208,11 +224,16 @@ ImagingTransverse(Imaging imOut, Imaging imIn) {
     int x, y, xr, yr, xx, yy, xxsize, yysize;
     int xxx, yyy, xxxsize, yyysize;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
     if (imIn->xsize != imOut->ysize || imIn->ysize != imOut->xsize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch(
+            "output size must be the transpose of the input size"
+        );
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -268,11 +289,14 @@ ImagingRotate180(Imaging imOut, Imaging imIn) {
     ImagingSectionCookie cookie;
     int x, y, xr, yr;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
     if (imIn->xsize != imOut->xsize || imIn->ysize != imOut->ysize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch("images must have the same size");
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -313,11 +337,16 @@ ImagingRotate270(Imaging imOut, Imaging imIn) {
     int x, y, xx, yy, yr, xxsize, yysize;
     int xxx, yyy, xxxsize, yyysize;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
     if (imIn->xsize != imOut->ysize || imIn->ysize != imOut->xsize) {
-        return (Imaging)ImagingError_Mismatch();
+        return (Imaging)ImagingError_Mismatch(
+            "output size must be the transpose of the input size"
+        );
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -784,8 +813,11 @@ ImagingGenericTransform(
     char *out;
     double xx, yy;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
 
     ImagingTransformFilter filter = getfilter(imIn, filterid);
@@ -841,8 +873,11 @@ ImagingScaleAffine(
     int xmin, xmax;
     int *xintab;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
 
     ImagingCopyPalette(imOut, imIn);
@@ -1028,8 +1063,11 @@ ImagingTransformAffine(
     double xx, yy;
     double xo, yo;
 
-    if (!imOut || !imIn || imIn->mode != imOut->mode) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imOut || !imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != imOut->mode) {
+        return (Imaging)ImagingError_ModeError("images must have the same mode");
     }
 
     if (filterid || imIn->type == IMAGING_TYPE_I16) {
diff --git a/src/libImaging/GetBBox.c b/src/libImaging/GetBBox.c
index 45fe717c5c2..53a549f06eb 100644
--- a/src/libImaging/GetBBox.c
+++ b/src/libImaging/GetBBox.c
@@ -152,7 +152,7 @@ ImagingGetExtrema(Imaging im, void *extrema) {
     FLOAT32 fmin, fmax;
 
     if (im->bands != 1) {
-        (void)ImagingError_ModeError();
+        (void)ImagingError_ModeError("image must have exactly 1 band");
         return -1; /* mismatch */
     }
 
@@ -242,7 +242,7 @@ ImagingGetExtrema(Imaging im, void *extrema) {
             }
             /* FALL THROUGH */
         default:
-            (void)ImagingError_ModeError();
+            (void)ImagingError_NotSupportedError("mode not supported");
             return -1;
     }
     return 1; /* ok */
@@ -304,7 +304,7 @@ getcolors32(Imaging im, int maxcolors, int *size) {
     }
 
     if (!im->image32) {
-        return ImagingError_ModeError();
+        return ImagingError_ModeError("image must have 32-bit data");
     }
 
     table = calloc(code_size + 1, sizeof(ImagingColorItem));
diff --git a/src/libImaging/Histo.c b/src/libImaging/Histo.c
index 7af60003511..8a405d68aa1 100644
--- a/src/libImaging/Histo.c
+++ b/src/libImaging/Histo.c
@@ -65,13 +65,13 @@ ImagingGetHistogram(Imaging im, Imaging imMask, void *minmax) {
     FLOAT32 fmin, fmax, scale;
 
     if (!im) {
-        return ImagingError_ModeError();
+        return ImagingError_ValueError(NULL);
     }
 
     if (imMask) {
         /* Validate mask */
         if (im->xsize != imMask->xsize || im->ysize != imMask->ysize) {
-            return ImagingError_Mismatch();
+            return ImagingError_Mismatch("mask and image size must match");
         }
         if (imMask->mode != IMAGING_MODE_1 && imMask->mode != IMAGING_MODE_L) {
             return ImagingError_ValueError("bad transparency mask");
@@ -98,7 +98,7 @@ ImagingGetHistogram(Imaging im, Imaging imMask, void *minmax) {
         } else { /* yes, we need the braces. C isn't Python! */
             if (im->type != IMAGING_TYPE_UINT8) {
                 ImagingHistogramDelete(h);
-                return ImagingError_ModeError();
+                return ImagingError_ModeError("image must have 8-bit data");
             }
             ImagingSectionEnter(&cookie);
             for (y = 0; y < im->ysize; y++) {
diff --git a/src/libImaging/Imaging.h b/src/libImaging/Imaging.h
index 20f74b519a3..891709971da 100644
--- a/src/libImaging/Imaging.h
+++ b/src/libImaging/Imaging.h
@@ -268,11 +268,15 @@ ImagingSectionLeave(ImagingSectionCookie *cookie);
 extern void *
 ImagingError_MemoryError(void);
 extern void *
-ImagingError_ModeError(void); /* maps to ValueError by default */
+ImagingError_ModeError(const char *message); /* maps to ValueError by default */
 extern void *
-ImagingError_Mismatch(void); /* maps to ValueError by default */
+ImagingError_Mismatch(const char *message); /* maps to ValueError by default */
 extern void *
 ImagingError_ValueError(const char *message);
+extern void *
+ImagingError_NotSupportedError(
+    const char *message
+); /* derives from NotImplementedError and ValueError */
 
 /* Transform callbacks */
 /* ------------------- */
diff --git a/src/libImaging/Matrix.c b/src/libImaging/Matrix.c
index 02676c9e31a..3eaa59e9bda 100644
--- a/src/libImaging/Matrix.c
+++ b/src/libImaging/Matrix.c
@@ -23,8 +23,11 @@ ImagingConvertMatrix(Imaging im, const ModeID mode, const float m[12]) {
     ImagingSectionCookie cookie;
 
     /* Assume there's enough data in the buffer */
-    if (!im || im->bands != 3) {
-        return (Imaging)ImagingError_ModeError();
+    if (!im) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (im->bands != 3) {
+        return (Imaging)ImagingError_ModeError("image must have exactly 3 bands");
     }
 
     if (mode == IMAGING_MODE_L) {
@@ -77,7 +80,9 @@ ImagingConvertMatrix(Imaging im, const ModeID mode, const float m[12]) {
             ImagingSectionLeave(&cookie);
         }
     } else {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_NotSupportedError(
+            "only conversions to modes L and RGB are supported"
+        );
     }
 
     return imOut;
diff --git a/src/libImaging/ModeFilter.c b/src/libImaging/ModeFilter.c
index 757cbc3fb86..99fed34c520 100644
--- a/src/libImaging/ModeFilter.c
+++ b/src/libImaging/ModeFilter.c
@@ -24,8 +24,11 @@ ImagingModeFilter(Imaging im, int size) {
     UINT8 maxpixel;
     int histogram[256];
 
-    if (!im || im->bands != 1 || im->type != IMAGING_TYPE_UINT8) {
-        return (Imaging)ImagingError_ModeError();
+    if (!im) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (im->bands != 1 || im->type != IMAGING_TYPE_UINT8) {
+        return (Imaging)ImagingError_ModeError("image must have 8-bit data in 1 band");
     }
 
     imOut = ImagingNewDirty(im->mode, im->xsize, im->ysize);
diff --git a/src/libImaging/Negative.c b/src/libImaging/Negative.c
index 114e8608e87..f8acb7293b9 100644
--- a/src/libImaging/Negative.c
+++ b/src/libImaging/Negative.c
@@ -24,7 +24,7 @@ ImagingNegative(Imaging im) {
     int x, y;
 
     if (!im) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ValueError(NULL);
     }
 
     imOut = ImagingNewDirty(im->mode, im->xsize, im->ysize);
diff --git a/src/libImaging/Offset.c b/src/libImaging/Offset.c
index 48c60b3c4f2..2506b637415 100644
--- a/src/libImaging/Offset.c
+++ b/src/libImaging/Offset.c
@@ -25,7 +25,7 @@
 Imaging
 ImagingOffset(Imaging im, int xoffset, int yoffset) {
     if (!im) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ValueError(NULL);
     }
 
     int xsize = im->xsize, ysize = im->ysize;
diff --git a/src/libImaging/Palette.c b/src/libImaging/Palette.c
index b2dacf656b5..4d7791b8aab 100644
--- a/src/libImaging/Palette.c
+++ b/src/libImaging/Palette.c
@@ -29,7 +29,9 @@ ImagingPaletteNew(const ModeID mode) {
 
     if (mode != IMAGING_MODE_RGB && mode != IMAGING_MODE_RGBA &&
         mode != IMAGING_MODE_CMYK) {
-        return (ImagingPalette)ImagingError_ModeError();
+        return (ImagingPalette)ImagingError_ModeError(
+            "only RGB/RGBA/CMYK images supported"
+        );
     }
 
     palette = calloc(1, sizeof(struct ImagingPaletteInstance));
diff --git a/src/libImaging/Paste.c b/src/libImaging/Paste.c
index 0396cf58095..4c74b409659 100644
--- a/src/libImaging/Paste.c
+++ b/src/libImaging/Paste.c
@@ -280,7 +280,7 @@ ImagingPaste(
     ImagingSectionCookie cookie;
 
     if (!imOut || !imIn) {
-        (void)ImagingError_ModeError();
+        (void)ImagingError_ValueError(NULL);
         return -1;
     }
 
@@ -290,12 +290,12 @@ ImagingPaste(
     ysize = (int64_t)dy1 - dy0;
 
     if (xsize != imIn->xsize || ysize != imIn->ysize || pixelsize != imIn->pixelsize) {
-        (void)ImagingError_Mismatch();
+        (void)ImagingError_Mismatch("image dimensions and pixel size must match");
         return -1;
     }
 
     if (imMask && (xsize != imMask->xsize || ysize != imMask->ysize)) {
-        (void)ImagingError_Mismatch();
+        (void)ImagingError_Mismatch("mask size must match image size");
         return -1;
     }
 
@@ -603,7 +603,7 @@ ImagingFill2(
     int sx0, sy0;
 
     if (!imOut || !ink) {
-        (void)ImagingError_ModeError();
+        (void)ImagingError_ValueError(NULL);
         return -1;
     }
 
@@ -613,7 +613,7 @@ ImagingFill2(
     ysize = (int64_t)dy1 - dy0;
 
     if (imMask && (xsize != imMask->xsize || ysize != imMask->ysize)) {
-        (void)ImagingError_Mismatch();
+        (void)ImagingError_Mismatch("mask size must match image size");
         return -1;
     }
 
diff --git a/src/libImaging/Point.c b/src/libImaging/Point.c
index dcf88daa586..ddca85b74c2 100644
--- a/src/libImaging/Point.c
+++ b/src/libImaging/Point.c
@@ -137,7 +137,7 @@ ImagingPoint(Imaging imIn, ModeID mode, const void *table) {
     void (*point)(Imaging imIn, Imaging imOut, im_point_context *context);
 
     if (!imIn) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ValueError(NULL);
     }
 
     if (mode == IMAGING_MODE_UNKNOWN) {
@@ -210,9 +210,12 @@ ImagingPointTransform(Imaging imIn, double scale, double offset) {
     Imaging imOut;
     int x, y;
 
-    if (!imIn || (imIn->mode != IMAGING_MODE_I && imIn->mode != IMAGING_MODE_I_16 &&
-                  imIn->mode != IMAGING_MODE_F)) {
-        return (Imaging)ImagingError_ModeError();
+    if (!imIn) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+    if (imIn->mode != IMAGING_MODE_I && imIn->mode != IMAGING_MODE_I_16 &&
+        imIn->mode != IMAGING_MODE_F) {
+        return (Imaging)ImagingError_NotSupportedError("only modes I/I;16/F supported");
     }
 
     imOut = ImagingNew(imIn->mode, imIn->xsize, imIn->ysize);
diff --git a/src/libImaging/Quant.c b/src/libImaging/Quant.c
index 99c2dbac3af..0713018e208 100644
--- a/src/libImaging/Quant.c
+++ b/src/libImaging/Quant.c
@@ -1678,7 +1678,7 @@ ImagingQuantize(Imaging im, int colors, int mode, int kmeans) {
     ImagingSectionCookie cookie;
 
     if (!im) {
-        return ImagingError_ModeError();
+        return ImagingError_ValueError(NULL);
     }
     if (colors < 1 || colors > 256) {
         /* FIXME: for colors > 256, consider returning an RGB image
@@ -1688,12 +1688,11 @@ ImagingQuantize(Imaging im, int colors, int mode, int kmeans) {
 
     if (im->mode != IMAGING_MODE_L && im->mode != IMAGING_MODE_P &&
         im->mode != IMAGING_MODE_RGB && im->mode != IMAGING_MODE_RGBA) {
-        return ImagingError_ModeError();
+        return ImagingError_NotSupportedError("only modes L/P/RGB/RGBA supported");
     }
 
-    /* only octree and imagequant supports RGBA */
     if (im->mode == IMAGING_MODE_RGBA && mode != 2 && mode != 3) {
-        return ImagingError_ModeError();
+        return ImagingError_ModeError("only octree and imagequant support RGBA");
     }
 
     // Hoisted here as these are invariant over the loops below.
diff --git a/src/libImaging/RankFilter.c b/src/libImaging/RankFilter.c
index fc8ce2db96d..07e34fffb47 100644
--- a/src/libImaging/RankFilter.c
+++ b/src/libImaging/RankFilter.c
@@ -64,8 +64,16 @@ MakeRankFunction(UINT8) MakeRankFunction(INT32) MakeRankFunction(FLOAT32)
     int x, y;
     int i, margin, size2;
 
-    if (!im || im->bands != 1 || im->type == IMAGING_TYPE_I16) {
-        return (Imaging)ImagingError_ModeError();
+    if (!im) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+
+    if (!im) {
+        return (Imaging)ImagingError_ValueError(NULL);
+    }
+
+    if (im->bands != 1 || im->type == IMAGING_TYPE_I16) {
+        return (Imaging)ImagingError_ModeError("image must be 1-band, 8-bpc");
     }
 
     if (!(size & 1)) {
@@ -121,7 +129,7 @@ MakeRankFunction(UINT8) MakeRankFunction(INT32) MakeRankFunction(FLOAT32)
     } else {
         /* safety net (we shouldn't end up here) */
         ImagingDelete(imOut);
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_ModeError(NULL);
     }
 
     ImagingCopyPalette(imOut, im);
diff --git a/src/libImaging/Reduce.c b/src/libImaging/Reduce.c
index 7e5634bed46..9269686a496 100644
--- a/src/libImaging/Reduce.c
+++ b/src/libImaging/Reduce.c
@@ -1454,7 +1454,9 @@ ImagingReduce(Imaging imIn, int xscale, int yscale, int box[4]) {
 
     if (imIn->mode == IMAGING_MODE_P || imIn->mode == IMAGING_MODE_1 ||
         imIn->type == IMAGING_TYPE_I16) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_NotSupportedError(
+            "mode P/mode I/16-bit images not supported"
+        );
     }
 
     /* Round the size up. Dividing before adding avoids overflowing for a
diff --git a/src/libImaging/Resample.c b/src/libImaging/Resample.c
index fee4cf8d9dc..5c3a6bcc8f3 100644
--- a/src/libImaging/Resample.c
+++ b/src/libImaging/Resample.c
@@ -758,7 +758,9 @@ ImagingResample(Imaging imIn, int xsize, int ysize, int filter, float box[4]) {
     ResampleFunction ResampleVertical;
 
     if (imIn->mode == IMAGING_MODE_P || imIn->mode == IMAGING_MODE_1) {
-        return (Imaging)ImagingError_ModeError();
+        return (Imaging)ImagingError_NotSupportedError(
+            "modes P and 1 are not supported"
+        );
     }
 
     if (imIn->type == IMAGING_TYPE_I16) {
@@ -779,7 +781,9 @@ ImagingResample(Imaging imIn, int xsize, int ysize, int filter, float box[4]) {
                 ResampleVertical = _ImagingResampleVertical_32bpc;
                 break;
             default:
-                return (Imaging)ImagingError_ModeError();
+                return (Imaging)ImagingError_NotSupportedError(
+                    "special image types are not supported"
+                );
         }
     }
 
diff --git a/src/libImaging/Storage.c b/src/libImaging/Storage.c
index 1c3f912f808..1259b9e5e44 100644
--- a/src/libImaging/Storage.c
+++ b/src/libImaging/Storage.c
@@ -748,7 +748,7 @@ ImagingNew2Dirty(const ModeID mode, Imaging imOut, Imaging imIn) {
         /* make sure images match */
         if (imOut->mode != mode || imOut->xsize != imIn->xsize ||
             imOut->ysize != imIn->ysize) {
-            return ImagingError_Mismatch();
+            return ImagingError_Mismatch("image modes and dimensions must match");
         }
     } else {
         /* create new image */