First, right after the body tag:
<img src='background.jpg' alt='' id='fixedbackground'>
Then in the CSS:
#fixedbackground {
position: fixed; /* The body will scroll over the image */
top: 0;
left: 0;
width: 100%; /* Scales to width of the body */
z-index: -1; /* Go BEHIND the body */
}
body {
background-color: #000; /* Choose a color that matches the bottom edge of the background image. Make the image match this, too, of course. */
}
An example: Leopard Creek Timber Frame