Ну сделай 2 элемента.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<style type="text/css">
*{margin:0;padding:0;cursor:none}
body {overflow:hidden;background:url('http://javascript.ru/forum/attachment.php?attachmentid=702&stc=1&d=1298243258') 50% 50%;}
#container {margin:50px auto; position:relative;overflow:hidden;background-image:url('http://javascript.ru/forum/attachment.php?attachmentid=702&stc=1&d=1298243258');height:277px;width:417px; }
#maskInner {
position:absolute;
border:800px solid #000;
filter: alpha(opacity=80);
-moz-opacity: 0.8;
-khtml-opacity: 0.8;
opacity: 0.8;
}
#maskOuter {position:absolute}
</style>
<script type="text/javascript">
window.onload=function(){
var mi=document.getElementById('maskInner').style;
var mo=document.getElementById('maskOuter').style;
var l=document.getElementById('container').offsetLeft;
document.onmousemove=function(e){
e||(e=window.event);
mi.cssText='left:'+(e.clientX-899-l)+'px;top:'+(e.clientY-903)+'px';
mo.cssText='left:'+(e.clientX-99)+'px;top:'+(e.clientY-53)+'px';
}
}
</script>
</head>
<body>
<div id="container">
<img src="http://javascript.ru/forum/attachment.php?attachmentid=708&stc=1&d=1298314949" id="maskInner">
</div>
<img src="http://javascript.ru/forum/attachment.php?attachmentid=707&stc=1&d=1298314949" id="maskOuter">
</body>
</html>