ok, just successfully done this via a quick port of a vB2 version I did for someone else the other week.
In the body section
locate the background box and add the url of the image so that it is in the following format
Code:
#E1E1E2 url(images/background13.jpg)
In the extra CSS attributes box place
Code:
background-attachment:fixed
You may wish for your page background to be transparent, so just delete the background from there.
That's it, you now have a fixed background.