You may be better off re-doing your images there. The way you have it, I don’t think you can get what you want with pure CSS. I’ve uploaded an example of what I mean HERE. It a free theme for TextPatteren I converted to pure HTML/CSS ages ago, when I was just learning my way around, but it illustrates a good technique for what you’re wanting to do I think. Take a close look at the "Home" area background and how the images are positioned and sized. You can download a zip of the site here if it helps illustrate, or just use FireBug.
Hope this helped!