First, you need to have a 'path' on the server to host the image. (ex: http://video.clankillas.net/images/spring.png).
Next you will want to make the Header-Background area Transparent. To do this: Admin CP -> Appearance -> Style Properties -> Header and Navigation -> Header ~ In the 'background' color-box, check 'clear value' at the bottom.
Finally you'll want to do this: Appearances/Styles/Style Properties/General/BODY ~ Make SURE that in the 'background' color-box, you have 'clear value' box (below) checked and then just put the path to where the image is, into the 'image' box to the right of the 'background' color-box.