If ICO image is smaller than (16, 16), save original size by default

Tests/test_file_ico.py

@@ -187,6 +187,15 @@ def test_incorrect_size() -> None:
             im.size = (1, 1)
 
 
+def test_save_1x2(tmp_path: Path) -> None:
+    im = Image.new("1", (1, 2))
+    outfile = tmp_path / "temp.ico"
+    im.save(outfile)
+
+    with Image.open(outfile) as reloaded:
+        assert_image_equal(im, reloaded)
+
+
 def test_save_256x256(tmp_path: Path) -> None:
     """Issue #2264 https://github.com/python-pillow/Pillow/issues/2264"""
     # Arrange
@@ -195,9 +204,9 @@ def test_save_256x256(tmp_path: Path) -> None:
 
         # Act
         im.save(outfile)
-    with Image.open(outfile) as im_saved:
+    with Image.open(outfile) as reloaded:
         # Assert
-        assert im_saved.size == (256, 256)
+        assert reloaded.size == (256, 256)
 
 
 def test_only_save_relevant_sizes(tmp_path: Path) -> None:
@@ -211,9 +220,14 @@ def test_only_save_relevant_sizes(tmp_path: Path) -> None:
         # Act
         im.save(outfile)
 
-    with Image.open(outfile) as im_saved:
+    with Image.open(outfile) as reloaded:
         # Assert
-        assert im_saved.info["sizes"] == {(16, 16), (24, 24), (32, 32), (48, 48)}
+        assert reloaded.info["sizes"] == {(16, 16), (24, 24), (32, 32), (48, 48)}
+
+    im2 = Image.new("1", (1, 1))
+    outfile = tmp_path / "temp.ico"
+    with pytest.raises(ValueError, match="All sizes too large for image"):
+        im2.save(outfile, sizes=[(2, 2)])
 
 
 def test_save_append_images(tmp_path: Path) -> None:

docs/handbook/image-file-formats.rst

@@ -452,8 +452,8 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
 **sizes**
     A list of sizes included in this ico file; these are a 2-tuple,
     ``(width, height)``; Default to ``[(16, 16), (24, 24), (32, 32), (48, 48),
-    (64, 64), (128, 128), (256, 256)]``. Any sizes bigger than the original
-    size or 256 will be ignored.
+    (64, 64), (128, 128), (256, 256)]``, or if it is smaller, only the image size.
+    Any sizes bigger than the original size or 256 will be ignored.
 
 The :py:meth:`~PIL.Image.Image.save` method can take the following keyword arguments:
 

src/PIL/IcoImagePlugin.py

@@ -57,15 +57,18 @@
 def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
     fp.write(_MAGIC)  # (2+2)
     bmp = im.encoderinfo.get("bitmap_format") == "bmp"
-    sizes = im.encoderinfo.get(
-        "sizes",
-        [(16, 16), (24, 24), (32, 32), (48, 48), (64, 64), (128, 128), (256, 256)],
-    )
+    if "sizes" in im.encoderinfo:
+        sizes = sorted(set(im.encoderinfo["sizes"]))
+    else:
+        sizes = (
+            [im.size]
+            if min(im.size) < 16
+            else [(d, d) for d in (16, 24, 32, 48, 64, 128, 256)]
+        )
     frames = []
     provided_ims = [im] + im.encoderinfo.get("append_images", [])
-    width, height = im.size
-    for size in sorted(set(sizes)):
-        if size[0] > width or size[1] > height or size[0] > 256 or size[1] > 256:
+    for size in sizes:
+        if size[0] > min(256, im.width) or size[1] > min(256, im.height):
             continue
 
         for provided_im in provided_ims:
@@ -90,6 +93,9 @@ def _save(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
             frame = provided_im.copy()
             frame.thumbnail(size, Image.Resampling.LANCZOS, reducing_gap=None)
             frames.append(frame)
+    if not frames:
+        msg = "All sizes too large for image"
+        raise ValueError(msg)
     fp.write(o16(len(frames)))  # idCount(2)
     offset = fp.tell() + len(frames) * 16
     for frame in frames: