Dist, it's not against the rules. I, and others, have had multiple images for signatures a lot of times, even checked in with the staff and yes it is definitely 100% legit. Rules state the total sum needs to be 50,000 bytes, nothing else. Normally images in signatures were allowed so we could have anime-banners, or site-banners/promos which are these small long strips. Of course now all you see is random anime characters everywhere.
You can even have a signature with both sides animated and with different timing to get the illusion of a longer loop; but the cases where you can make use of that are rare.