Head:
<script language="JavaScript1.2" type="text/javascript">
function ChangeBgImg(imgs) {
bdy[0].style.backgroundImage = 'url('+imgs+')';
}
function ChangeBgImgInit() {
bdy = document.getElementsByTagName("body");
bdy[0].style.margin = "10px";
bdy[0].style.backgroundImage = 'url(../p25big-assed-backgrounds/babg-lightcircle.jpg)';
}
onload=ChangeBgImgInit;
</script>
Body:
<a href="javascript:;" onclick="ChangeBgImg('web.jpg');return false">Background 1</a>
<a href="javascript:;" onclick="ChangeBgImg('lightcircle.jpg');return false">Background 2</a>