[assassingod]

Right, let's see (assuming you want to keep the usualy style for each for but different header)

Download your current style, upload it but make a new style, and modify your header. Then edit your forum so it uses your modified header style. and do that for each for

OR

You can add to your header template an image, and name all the images you want to use 1,2,3 or 4 etc (to relate to a forum and make sure they are the same extension) then use $forum[forumid] for the image name (i.e <img src="{imagesfolder}/$forum[forumid].gif)

First method is untested and should work
Second method is untested and MAY work.....