Adding a background image is done in the stylesheet.
This is a sample from the stylesheet in use on the Fan Odyssey site in my sig. Note the lines I highlighted in red.
Code:
/*wrappers - page or section containers*/
#mainWrapper {
background-color: #000000;
background-image: url('../images/space011.jpg');
text-align: left;
width: 100%;
vertical-align: top;
}
#headerWrapper, #contentMainWrapper, #logoWrapper, #cartBoxListWrapper, #ezPageBoxList, #cartBoxListWrapper ul, #ezPageBoxList ul, #mainWrapper, #popupAdditionalImage, #popupImage {
margin: 0em;
padding: 0em;
}
#logoWrapper{
background-image: url('../images/space011.jpg');
background-repeat: repeat-x;
background-color: #000000;
height:110px;
width: 100%;
}