Setting a full screen background image

[Objectivity]

I'm pretty sure this is a simple question, but I've searched for awhile and can't find how to do it.

I want to set a background image on my website that only appears outside the fixed-width of the website edges. I know how to set a background image to repeat, but having a static, one-time image is baffling me. Is it one image? Or is it a separate image for the left and right?

The style I'm looking for is similar to the ads that appear on www.gamespot.com and other sites that take advertising like that. The only difference is that I don't need it to coordinate with content inside the web window.

Can someone enlighten me?

[Medyman]

html, body {
   margin:0;
   padding:0;
   background-image:url(relative/path/to/image);
   background-repeat:no-repeat;
   background-attachment:fixed;
}

#container {
   margin:0 auto;
   width:900px; /* your fixed width */
}