It won't. The idea is that the background image would only spread as far as the site content. To force the footer to the bottom see the link I provided in my fist reply

but I wouldn't recommend doing that anyhow it would leave great voids of white space.
The other method I mentioned aims to limit the design height to that of the content.. for example here is your site with the background image filling the page even with no content:
http://img160.imageshack.us/img160/5387/nowq2.jpg
And an example of limiting the style to the content:
http://img479.imageshack.us/img479/8924/yesnp2.jpg