CSS n00b question: Trouble with Divs stretching the full width of the browser

Hey everyone,

I’m working on a website for someone, and I’ve decided to try my hand at using the 960 grid system. CSS layouts are definitely not my area of expertise, and I’m running into some trouble when the browser window size is reduced.

The most recent version of this work in progress can be found here:
http://sol.vectortrance.com/

If you are viewing it minimized browser window, and the viewing area is less than 960px, you’ll notice that when you use the horizontal scrollbars to see the content, the main header graphic no longer reaches the edge of the window.
Same thing with the footer graphic.

I’m sure it’s probably a simple oversight, or a simple solution, but could anyone please offer some advice on how to rectify this ?