I use a background image on the Fan Odyssey site in my sig. Here's a snippet from the stylesheet for that site.

Code:
/*wrappers - page or section containers*/
#mainWrapper {
	background-color: #000000;
  background-image: url('../images/space011.jpg');
	text-align: left;
	width: 100%;
	vertical-align: top;
	}
Have you played around with it in Firefox?