Backstretch is a simple jQuery plugin that allows you to add a dynamically-resized background image to any page. The image will stretch to fit the page, and will automatically resize as the window size changes.
Demo
Documentation
Development
Info
Current Version: 1.2.4
Compressed File Size: < 2K
License: MIT/GPL
Demo
jQuery backstretch is being used on this page. Resize the window to see backstretch in action.
<script type="text/javascript">
$.backstretch("http://farm3.static.flickr.com/2443/3843020508_5325eaf761.jpg");
</script>
Additionally, there are some examples included with the plugin download itself, including onclick image swapping and a slideshow.
Source