TryNinja is right. It is possible to use only the avatar in APNG format. I don't have a chance to test it right now. Perhaps theymos was off already and the APNG format.
I dont know if you noticed but I literally quoted a post where theymos states that he made the system detect and disallow APNGs images. My point is that this was possible just because there was no system to detrct it, but it was not intended to be allowed.