diff --git a/docs/handbook/image-file-formats.rst b/docs/handbook/image-file-formats.rst
index a12de82e2c0..7bb846b2a3e 100644
--- a/docs/handbook/image-file-formats.rst
+++ b/docs/handbook/image-file-formats.rst
@@ -200,12 +200,16 @@ attributes before loading the file::
 ICNS
 ^^^^
 
-Pillow reads and (macOS only) writes macOS ``.icns`` files.  By default, the
+Pillow reads and writes macOS ``.icns`` files.  By default, the
 largest available icon is read, though you can override this by setting the
 :py:attr:`~PIL.Image.Image.size` property before calling
 :py:meth:`~PIL.Image.Image.load`.  The :py:meth:`~PIL.Image.Image.open` method
 sets the following :py:attr:`~PIL.Image.Image.info` property:
 
+.. note::
+
+    Prior to version 7.2.0, Pillow could only write ICNS files on macOS.
+
 **sizes**
     A list of supported sizes found in this icon file; these are a
     3-tuple, ``(width, height, scale)``, where ``scale`` is 2 for a retina
diff --git a/src/PIL/IcnsImagePlugin.py b/src/PIL/IcnsImagePlugin.py
index 660de2b9250..cf9b5dcf5e8 100644
--- a/src/PIL/IcnsImagePlugin.py
+++ b/src/PIL/IcnsImagePlugin.py
@@ -300,13 +300,12 @@ def load(self):
         self.load_end()
 
 
-def to_int(s):
+def _to_int(s):
     b = s.encode("ascii")
     return (b[0] << 24) | (b[1] << 16) | (b[2] << 8) | b[3]
 
 
-MAGIC = to_int("icns")
-HEADER_SIZE = 8
+MAGIC = _to_int("icns")
 TOC = "TOC "
 
 
@@ -326,32 +325,34 @@ def _save(im, fp, filename):
     file_size = 0
     entries = []
     provided_images = {im.width: im for im in im.encoderinfo.get("append_images", [])}
-    for index, s in enumerate(sizes):
-        temp = io.BytesIO()
+    temp_sizes = {s: io.BytesIO() for s in set(sizes)}
+    for s, temp in temp_sizes.items():
         nb = provided_images[s] if s in provided_images else im.resize((s, s))
         nb.save(temp, "png")
+    for index, s in enumerate(sizes):
+        temp = temp_sizes[s]
         file_size += len(temp.getvalue())
         entries.append(
             {"type": size_str[index], "size": len(temp.getvalue()), "stream": temp}
         )
 
     # Header
-    fp.write(struct.pack("i", MAGIC)[::-1])
-    fp.write(struct.pack("i", file_size)[::-1])
+    fp.write(struct.pack("<i", MAGIC)[::-1])
+    fp.write(struct.pack("<i", file_size)[::-1])
 
     # TOC
-    toc_size = HEADER_SIZE + (len(entries) * HEADER_SIZE)
-    fp.write(struct.pack("i", to_int(TOC))[::-1])
-    fp.write(struct.pack("i", toc_size)[::-1])
+    toc_size = HEADERSIZE + (len(entries) * HEADERSIZE)
+    fp.write(struct.pack("<i", _to_int(TOC))[::-1])
+    fp.write(struct.pack("<i", toc_size)[::-1])
     for e in entries:
-        fp.write(struct.pack("i", to_int(e.get("type")))[::-1])
-        fp.write(struct.pack("i", HEADER_SIZE + e.get("size"))[::-1])
+        fp.write(struct.pack("<i", _to_int(e["type"]))[::-1])
+        fp.write(struct.pack("<i", HEADERSIZE + e["size"])[::-1])
 
     # Data
     for index, e in enumerate(entries):
-        fp.write(struct.pack("i", to_int(e.get("type")))[::-1])
-        fp.write(struct.pack("i", HEADER_SIZE + e.get("size"))[::-1])
-        fp.write(e.get("stream").getvalue())
+        fp.write(struct.pack("<i", _to_int(e["type"]))[::-1])
+        fp.write(struct.pack("<i", HEADERSIZE + e["size"])[::-1])
+        fp.write(e["stream"].getvalue())
 
     fp.flush()