It can be done, in a few ways.
CSS isn't the easiest, or the most possible!
Here's a solution I took off another site.
If you don't understand it, or don't know how to, just write back.
code:
<html>
<head>
<title>Random Backs</title>
<HEAD>
</HEAD>
<script language="JavaScript">
var theImages = new Array();
theImages[0] = '1.gif'
theImages[1] = '2.gif'
theImages[2] = '3.gif'
theImages[3] = '4.gif'
// and so on...
var j = 0
var p = theImages.length;
var preBuffer = new Array()
for (i = 0; i < p; i++){
preBuffer = new Image()
preBuffer.src = theImages
}
var whichImage = Math.round(Math.random()*(p-1));
document.write('<body background="'+theImages[whichImage]+'">');
</script>
</body>
</html>
