I have found one interesting stuff in jQuery, to resize the image with window dimension. ie if you want to fit the background image to the screen what ever the window size or screen resolution, just use it this snippet.

View Demo Download Source

JQuery

<script src="jquery.backstretch.min.js"></script>
<script type="text/javascript">
$(document).ready(function() {
    $.backstretch("http://your/image/path");
});
</script>

You can download the plugin files here