Why is my background image looking like this?

Attached are two screenshots of a page that I built (http://www.mms-nashville.com/index.php). This first one represents what I intend the site to look like regardless of the size of the browser:


Here's what the same page looks like in a smaller browser:


What do I need to change in my CSS so the smaller browser scenario doesn't have that hideous white space going on on the left hand side?

My stylesheet is at http://www.mms-nashville.com/stylesheet.css

Thanks!