Place your html image code inside the forum description inside the forum manager which is in the admincp.
Image code: This is what I use. I also add some text but didn't include it here. You can add your sizes. I do this to make the banners expand when the forum is opened.
PHP Code:
<a href="########" rel="nofollow" target="_blank"><img src="###########" height="auto" width="100%" border="0" alt="######">