BACKSTRETCHING:

For more info check here:
http://srobbin.com/jquery-plugins/backstretch/

In short, add the following code to the head of your html page, and save "bgimage.jpg" in the same folder as your html page.

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script type="text/javascript" src="http://jessicahische.is/awesome/jquery.backstretch.min.js"></script>
<script type="text/javascript">
$.backstretch("bgimage.jpg", {speed: 150});
</script>