Okay, never ran into this one before. I made an edit to each instance of forum_old.gif and forum_new.gif using the replacement option in the Admin CP with forum_old.png and forum_new.png. The edits went fine, but the pages insist on trying to load the GIFs when loaded, except in the legend at the bottom of the page. I'm at a loss as to why that is and what I can do to change it. This is on a hack free install, by the way.