sry
Make sure that it's 150x150px or under, otherwise the server will resize it and kill its animutationism (that's what happened here). If one of the dimensions is exactly 150px, try bumping it down to 149px to see if that helps.
Another possibility is that you might also have to have it hosted elsewhere for it to work, like Photobucket or whatever. I think SMF processes all uploaded avatars.