I've never used XenForo but I'm reading that the header banner is part of the
logo_block
section in the site template by default.
Wherever it is, setting the image itself should be as easy as inserting a number inline at the image source, randomly:
PHP:
src="/forum/data/assets/logo/banner<? rand(0, $number_of_banners) ?>.gif"
or, exhaustively different based on the current day:
PHP:
src="/forum/data/assets/logo/banner<? ((date('z') + 1) % $number_of_banners) ?>.gif"
etc. (mind the file type if they're not all gifs).
If you want to source images automatically from a thread, or otherwise avoid updating the banner count with every addition, there's a bit more to it, but as long as you can customize the page source it should be trivial. If you already knew all that and the issue is neatly setting multiple images as logos specifically, I don't know if you can do that in XenForo but there should be many workarounds.