Try to put the script below anywhere in your skin.html or your skin.ascx.

<script type="text/javascript">
var mRandomBackgrounds = 3; // Set how many many different backgrounds you want to have
var mClassPrefix = 'bodybg'; // CSS class prefix. You can leave it as is.
var mRandomNumber = Math.round((mRandomBackgrounds-1)*Math.random()) + 1;
var mBody = document.getElementsByTagName('body')[0];
mBody.className = mClassPrefix + mRandomNumber;
</script>

and then in your skin.css

.bodybg1{background:url(image1.jpg);}
.bodybg2{background:url(image2.jpg);}
.bodybg3{background:url(image3.jpg);}