All you have to do is find the image file for the banner, save a copy to disk, edit it the way you want but try to avoid resizing it, save it to the image folder in the folder the style you are using is it. Use the same name as the banner image file already there. You should then find the banner image changes on your forum.

For an example, take a look at my new forum at http://aerialfreeviewforum.bizhat.com/.