Add append_images support for ICO

Tests/test_file_ico.py

@@ -85,6 +85,24 @@ def test_only_save_relevant_sizes(tmp_path):
         assert im_saved.info["sizes"] == {(16, 16), (24, 24), (32, 32), (48, 48)}
 
 
+def test_only_save_append_images(tmp_path):
+    """append_images should work to provide alternative sizes"""
+    im = hopper()
+    provided_im = Image.new("RGBA", (32, 32), (255, 0, 0, 255))
+    outfile = str(tmp_path / "temp_saved_multi_icon.ico")
+    im.save(outfile, sizes=[(32, 32), (64, 64)], append_images=[provided_im])
+
+    with Image.open(outfile) as reread:
+        reread.size = (64, 64)
+        reread.load()
+        assert_image_equal(reread, hopper().resize((64, 64), Image.LANCZOS))
+
+    with Image.open(outfile) as reread:
+        reread.size = (32, 32)
+        reread.load()
+        assert_image_equal(reread, provided_im)
+
+
 def test_unexpected_size():
     # This image has been manually hexedited to state that it is 16x32
     # while the image within is still 16x16

src/PIL/IcoImagePlugin.py

@@ -52,6 +52,7 @@ def _save(im, fp, filename):
     sizes = list(sizes)
     fp.write(struct.pack("<H", len(sizes)))  # idCount(2)
     offset = fp.tell() + len(sizes) * 16
+    alt_images = {im.size: im for im in im.encoderinfo.get("append_images", [])}
     for size in sizes:
         width, height = size
         # 0 means 256
@@ -63,9 +64,12 @@ def _save(im, fp, filename):
         fp.write(struct.pack("<H", 32))  # wBitCount(2)
 
         image_io = BytesIO()
-        # TODO: invent a more convenient method for proportional scalings
-        tmp = im.copy()
-        tmp.thumbnail(size, Image.LANCZOS, reducing_gap=None)
+        if size in alt_images:
+            tmp = alt_images[size]
+        else:
+            # TODO: invent a more convenient method for proportional scalings
+            tmp = im.copy()
+            tmp.thumbnail(size, Image.LANCZOS, reducing_gap=None)
         tmp.save(image_io, "png")
         image_io.seek(0)
         image_bytes = image_io.read()