<html>
<head>
<style type='text/css'>
#elem{
height: 100px;
width: 100px;
}
</style>
<script type='text/javascript'>
function cBg(){
document.getElementById('elem').style.background = "#828484 url(bg.png)";
}
</script>
</head>
<body>
<div id='elem'></div><br/>
<input type='button' value='Click' onclick='cBg()'/>
</body>
</html>