Auto-resizable Background

/* Use this markup: */

<div id="background">
<img src="img.jpg" class="stretch" alt="" />
</div>

*/ with the following css: */

#background {
    width: 100%;
    height: 100%;
    position: absolute;
    left: 0px;
    top: 0px;
    z-index: 0;
}

.stretch {
    width:100%;
    height:100%;
}