Changing Background Image at RunTime

Ok so this is so so simple yet I cannot remember how it is done - shoot me!

lets say for instance I had a button that when clicked, changed the background image for the main document.

Thus allowing the user to choose a background image they want for my site.

Any help

 

Here's an example that changes the background to the google logo. Just use a variable instead of the direct link to change it to what the user selected.

<script type="text/javascript">
function changeBg()
{
	
	document.body.background = 'http://www.google.com/intl/en/images/logo.gif'
}
</script>
<input type="button" value="change" onClick="changeBg();" >

Code (markup):

 

you can also change the className of the body with the onClick event and in the new class name, have a different background image.