Container floating over a background image?

I like the look of this site but I can’t figure out how you set up a container of content centered and hovering above a background image, so that the container stays centered as the browser window width is changed, and the background seems to repeat as the browser window gets wider. I looked at the source with firefox and see how the containers are set up. I deduced from image properties that the background image is called bg3.jpg, but don’t see it used in the page source.

Wrong image? I see a graph.

Crap mo’ stank, yeah, wrong link.

It can be configured in CSS as a fixed attachment. The container would use Auto margins.