Background image on main page

OK, being completely hopeless at these things, I really need the help of you guys.

If you look at my site: http://www.greenfieldit.com.au/shop, you will see that I have removed the side boxes and want to use just my shop front (with door) image as the background, so that I can (in theory) overlay the sideboxes, so they fit within the windows of the graphic. I'm not sure how this could be done; at the moment, I have just dumped the image file in my main page define_main_page.php. Ideally I would still like to have the categories and a few other boxes on the main page.

Any ideas?

For balance I would make the door a double door and shrink the windows and these could become the vessel for the side boxes.

Some things might have to be fixed so as to not break the look but not knowing how much product you will have makes it hard to assertain