You can always try a CSS trick but it only works with a 100% x 100% html/body.
CSS:
<style type="text/css">
html, body {margin:0; padding:0; width:100%; height:100%; overflow:hidden;}
#background{position:absolute; z-index:1; width:100%; height:100%;}
}
</style>
HTML
<body>
<div>
<img id="background" src="YOUR REAL IMAGE URL" alt="xxx" title="xxx" />
</div>
......and the rest of the HTML after this