I just realized that I had all of the Arts/Forms, Founders, and House icons conveniently on my computer already, and it only took me a couple minutes to convert them all to avatars. So, now if you go to ucp.php?i=profile&mode=avatar (that's the User Control Panel>Profile>Edit Avatar) then click on the Display Gallery button at the bottom, you'll see a pull-down menu with these options: ArMArtsForms, ArMCovers, ArMFounders, ArMHouses.
Note that this may mess up anybody who already chose one of Erik Dahl's cover avatars, since they're in a different folder now for organization's sake, so you may need to re-select your avatar. Sorry about that!