Rzaxan,
и что не так?
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
body, html{
height:100%;
}
#clock{
position:fixed;
top:0;
left:0;
height:100%;
background-image:url(../rek/clock.jpg);
background-size:100% 100%;
background-color: #CCCCCC;
}
</style>
</head>
<body>
<div id="clock"></div>
<script>
var x = screen.width;
var xx = (x-970)/2-15;
document.getElementById('clock').style.width = xx+'px';
</script>
</body>
</html>