theymos mentioned that he may remove them someday but since he's too busy, it will take a while:
OK, APNGs are now detected. Old ones are grandfathered for now, in part because people have ongoing campaigns, but in the future I might (or might not) go through and remove them.
Stake Campaign's remaining animated avatars are easy to find though, all you have to do is look at the spreadsheet and check each participant's profiles.
If you think that it's really annoying, then make a thread with list of users with annoying animated avatars and report it to the admin for manual removal.