You do realize that by making this thread and calling attention to the fact that you can have animated profile pictures you have likely prompted more people to make them.
Not sure if it's possible anymore.
Ordinary image attachments are certainly mangled by the forum, and this breaks animated GIFs.
Maybe I should try, purely for science
edit
Yep, I can upload a GIF avatar but the damn thing isn't moving
That being said, I can apparently use any existing GIF present on the forum or elsewhere...
There is no (reasonable) technical way to prevent this, save for disabling externally hosted avatars.