Java script: can't change background image

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Hi everyone,
I'm trying to select an image for a div's background whereby JavaScript as follows:

<!DOCTYPE html>
<html>
<head>
<style>
div {
    width: 400px;
    height: 400px;
    border: 1px solid black;
}
</style>
<script>
		function ChangeImage(param)
		{ 
			switch(param)
			{
				case '1':
					return("http://localhost/tlushim/images/greensilver.gif");
					break;
				case'2':
					return("http://localhost/tlushim/images/greensilver1.gif");
					break;     
			}
		}
	</script>
</head>
<body>
	<div id="myDiv"></div>
	<script>
    	document.getElementById("myDiv").style.backgroundImage = "url(ChangeImage('1'))";
	</script>
</body>
</html>

But I dont get any background image ! Can anyone see what's wrong with my code?
Thanks