Thank you so much for the response. I think I can make sense of this.
How does this look?
<script type="text/javascript">
var path = '/';
var images = new Array('mainpic1.jpg','mainpic2.gif','mainpic3.jpg','mainpic4.gif);
var number = Math.random();
var image = images[1 + Math.round(number * 3)];
document.write('<img src="'+path+image+'" BORDER=0 HEIGHT=400 WIDTH=240);
</script>