На обычном JavaScript:
<script type="text/javascript">
function loadBackgroundImage() {
document.body.style.backgroundImage="url(background.jpg)";
}
</script>
HTML:
<body onload="loadBackgroundImage();">
В jQuery:
<script type="text/javascript">
$(document).ready(function() {
$("body").css("background-image","url(background.jpg)");
});
</script>