you can do this a few ways, one is with template edits. notices & forum blocks.
regardless you will have to turn the banners you have into links. This is how banners are created.
You can test this easily by, uploading your banners to your domain, or other file sharing sites, and turn them into links with easy html/
Just fill in url here & url of image... -leave the quotes. You can test that by taking that code and filling it into the HTML box of a notice to see how it is done.
<a href="URL HERE"><img src="URL OF IMAGE"></a>
as well just putting this code into notices, there are also, numerous "adsense" mods around. or other advertising plugins, these can be used for more then adsense, if you would like you can use those mods, to simply put your own ad codes in like the above ones and can promote you forum space to paying advertisers...your friends websites...your other sites..etc..
but you will always need to turn a picture into a link to create a banner.. Very simple with the above code.