Hi,
You didn't say if it is a color or a wallpaper image for background. Most HTML elements can have the attribute BGCOLOR, eg.
bgcolor="#3367cd"
That's the color code for the blue at the top of the Forum page behind the Auctiva logo.
Do you have any code you've created that can be used as a base? If so, copy and paste it to the practicewriter at www.practicewriter.com and post back the link it provides.
Danno