change header image by nodetype

I am working on my first attempt at modifying a theme and I have what I think is a simple question.

Firstly I am using the Blue Business theme [http://www.alldrupalthemes.com/bluebusiness.html] which I have widened a little, but so far that’s the only real change I have needed to make.

If you have a look at http://community.afterthemouse.com/ you’ll notice the large banner picture of a cloud. What I would like to do is…

For each type of node, Forum, User Profile, Image Gallery etc. I’d like to “float” different information into the space now occupied by the cloud image. So for the Forum example I might have an image symbolising communication etc.

How do I change the image in the div that presently holds the cloud picture?

Many thanks

Mouse