Only moderators can approve or deny avatars. When an avatar is denied, you're left with nothing. The easiest thing to do, then, is ask a moderator to deny your avatar, and then reload it. If you time it right, this works. If there are too many moderators online, another one might approve it before the one you IMed can deny it. Plan accordingly.
Or you can always upload this one:
The moderators have been instructed to deny it when they see it, and you will be left with no avatar at all.