You could replace this in your header template
HTML Code:
<img src="$stylevar[titleimage]
with this
HTML Code:
<img src="./board/images/logo/logo-$forum[forumid].jpg" />
Then you would have a folder called /logo/ with
logo-1.jpg (for forum id 1)
logo-2.jpg (for forum id 2)
etc
I'm just wondering if there would be an easy way of adding a template conditional to show the default header if a customized one wasn't available but I think that would require adding a field to the db and marking it off to determine if one is available or not.