Hintergrundbild automatisch anpassen

<script type="text/javascript">
window.onload = function () {
  var w = document.body.clientWidth;
  var h = document.body.clientHeight;
  document.getElementById('hgimg').style.width = w + 'px';
  document.getElementById('hgimg').style.height = h + 'px';
}
</script>
<style type="text/css">
<!--
table, font, b, i, script, img, div { z-index:10; position:fixed;}

div#hg { z-index:0; position:absolute; left:0px; top:0px;}
img#hgimg { z-index:0;}
-->
</style>

<div id="hg"><img id="hgimg" src="" alt="" /></div>