Center Site on background

I am designing a site and I have created a background that needs to stay centered with the site as the browser width is expanded and contracted as on: [login to view URL] This is what I have so far: [login to view URL] But as you can see when you expand and contract the browser, the backgroun image moves and looks wierd. I need some one to either do this for me or show me how to do it myself or both. Budget: $30