Why does my Steam artwork GIF not move?

Steam re-saved the file after you uploaded it, and the new copy only shows the first frame. Changing the last byte of the GIF from 3B to 21 stops Steam re-saving it.

Does the byte change break the GIF?

No. Browsers ignore it and play the GIF as normal. Only Steam treats the file differently.

What is the file size limit for Steam artwork?

5 MB per file. The Artwork Maker keeps every piece under 5,000,000 bytes, because Steam doesn't say whether it means 5,000,000 or 5,242,880.

Why Steam shows your GIF as a still image

Steam re-saves some GIFs after upload, and the re-saved copy only shows its first frame. A one-byte change to the file stops that.

What's going on

When you upload artwork or a Workshop item, Steam processes the file. For some GIFs it writes a new copy, and that copy plays as a still image on your profile. Steam doesn't say exactly when this happens. Bigger files seem to be the usual trigger, which is why keeping files small helps.

The fix

Every GIF ends with one byte, 3B, that marks the end of the file. Change it to 21 and browsers still play the GIF fine, but Steam sees a file it doesn't expect and leaves it alone. Your upload stays animated.

The Artwork Maker does this to every piece. It's the Steam animation fix setting under Advanced settings, and it's on by default.

To do it by hand, open the GIF in a hex editor, go to the very end of the file, change the last 3B to 21 and save.

If it still freezes

Lower the frame rate. Fewer frames means a smaller file. The Artwork Maker uses 18 fps by default. Try 12 or 15 and make the pieces again.
Check the fix is on. Look under Advanced settings before you make the pieces.
Don't re-save the file. Opening a piece in an image editor and saving it writes a normal end byte back, which undoes the fix. Upload the file you downloaded.
Stay under 5 MB. Steam won't take artwork over that. The Artwork Maker lowers the colours until every piece fits.

Open the Artwork Maker

Questions

Why does my Steam artwork GIF not move?: Steam re-saved the file after you uploaded it, and the new copy only shows the first frame. Changing the last byte of the GIF from 3B to 21 stops Steam re-saving it.
Does the byte change break the GIF?: No. Browsers ignore it and play the GIF as normal. Only Steam treats the file differently.
What is the file size limit for Steam artwork?: 5 MB per file. The Artwork Maker keeps every piece under 5,000,000 bytes, because Steam doesn't say whether it means 5,000,000 or 5,242,880.