try something like this

<HTML>
<HEAD>
<SCRIPT LANGUAGE=javascript>
<!--

function window_onresize() {
myImg.height = window.document.body.clientHeight
myImg.width = window.document.body.clientWidth
}

//-->
</SCRIPT>
</HEAD>
<BODY onload="return window_onresize()" onresize="return window_onresize()">
<img id=myImg src="coffee.gif" style="position:absolute;left:0px;top:0px;">
</BODY>
</HTML>