Yes, in your header where you have the <img src for your banner, you'd just need to surround that with a link to your main forum.

So it would look like:

Code
<a href="http://www.clubadventist.com">
<img src="http://clubadventist.org/forum/banner/newcafall2009.gif" width="730" height="190" alt="Club Adventist" border="0">
</a>