Changing the actual forums background

[StarBurst]

I've been trying to add an image as the background for my forum for some time now with no suceeding.

Each time I change the background to the image I want, the border around the edge of the forum changes to the image, but the actual baackground of the forum doesn't. Is there any way of setting an image as the background visible behind the chatbox, forums, logo etc?

Thanks in advance for any help!