There's a lot in the thread that misty's pointed you towards about coding solutions for changing template or loading different flash files. But those parts of the thread are more than you need.

The answer best suited to what you want to do however, is still in that thread. I recommend taking at look at post #2 which shows the naming convention for having different stylesheets for different pages. As your header image is defined in your stylesheet, having a succession of very short stylesheets that simply over-ride that image with the name of the one you want for a particular page is the quickest and easiest way of achieving what you want.