*raises and shakes his cane at a passing cloud*
I haven't looked at the feature in some time, but there is the banner advertising option in the ACP. It could be used for rotating banners, but separate from the header. Could also embed a rotating image in a custom block that could be placed above or below the forums, or on the sidebar.
A sidebar ad block could have images for upcoming events, or satire, or recruitment, or.....*shrugs* "TWPAF Dragon Pirates Need YOU. Join Today!", hyperlinked to the signup thread.