You need to be a bit more specific with your question.
If you want to use a background image, you add it to the correct place(s) in the stylesheet. Here's an example fromt the stylesheet used in the Fan Odyssey site in my sig.
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%;
}