Animated images were allowed to use as avatars but because they are annoying images, theymos disabled it. After that announcement, you can not upload and use animated images for avatar.
Years ago, newbies can wear avatars too but after a forum hack, it was disabled. There are glitches and you sometimes can see newbies wear avatars (like this user
whistle11 or more newbies in
that thread) or users with animated avatars like
yahoo62278. They will lose those avatars if they remove it or upload a new image.
I've had my avatar for years now, you are correct that if I remove it I lose it as well. That is why any signature campaign that wants me has to agree that my avatar space is able to stay as is. I would remove it, but a company will have to open their wallets big for that to happen.