For a fixed background you just use the "fixed" CSS attribute. Also at quick glance there are several issues on your site I can see right away.
- The background image is very large
- If you view the site in a wide monitor you see the edges of the image
- The "footer" background should be part of the body and not the footer for what I believe you are trying to do.
Those are a few things I saw but to correct all of that would take multiple hours of working with images and CSS positioning. As a matter of fact I just got through with working on a site for a client doing just that sort of stuff and I put 6+ hours into getting things placed properly so that the site worked in multiple browsers.
How good are you at CSS? How much do you know about images sizes and resolutions for the web? Those are two big areas that you will need to know to accomplish what you want with this site. It's a whole lot easier if you install Atahualpa with one of the default styles and then start the design process with the theme in mind rather than trying to make the theme try to duplicate a picture.