Billionfold said it best.

Quote Originally Posted by Billionfold
Note: The maximum size of your custom image is 100 by 100 pixels or 39.1 KB (whichever is smaller).

Mmk I'm high so I'll try and explain the best I can. I think the reason you gifs are unanimated after you upload them, is because the the file size is over 39.1, but the image size isn't over 100x100. So that means you can use the avatar, but because it is animated and the file is too big, it won't show animated on here.

(If that didn't make sense, someone please explain it better.)