Theme background change for image

Hey peeps!

I have been trying to figure out how to change the background of my wordpress theme to an image file instead of the current color that is there… I know there are several posts here on how to do this, but I have tried what is being suggested and only come up with the same result… which is not the required background change.

If you have a look at mu site (which by the way is still being experimented on) http://www.mad-bean.com I’m trying to effect the very back background… everything else I have tried has only effected the dark grey piece in the middle of the screen instead of the background that fills the screen.

Any advise?