How to fix background image

[Goonie]

I'm having some trouble trying to make my background image fixed.

I found this: http://www.vbenhanced.com/forums/hin...letin-4-a.html

I did exactly what it said. I found body_background in StyleVars and I added 'url(images/ReptileGreen/elements/background.jpg) fixed center' to Background Image.
It didn't fix the forums background image but did add an extra layer of background which became fixed (see attached image - the part I have circled wasn't there until I added the image details in the StyleVars and that circled part is fixed, which isn't what I wanted).

How do I make it so the WHOLE background becomes fixed and I don't get an extra layer of background.

I hope this makes sense. Thanks very much.