[Phantasmagoric]

I think this is what you are looking for:


The background image is not resizing, it just looks that way due to the gradient, the forum is fixed width and the white within the centre of the background image matches the fixed width of the forum.

First off, set your forums width:

In the main css find this:
Main Table Width (pixels or percentage)
(This is the width of the table that contains all your vBulletin content)

In the data entry field add 800 (or 960 or what ever width you want, do not put 800px, just 800)


Create your background image and it bg.jpg (for this examples purpose), upload your background image to your skins misc folder

in the main css of your skin find:

Body (Customized in this Style)

Inline find:
Standard CSS Attributes
Background

In the data entry field overwrite existing content with this (change the #221F18 to what ever the colour is of the very last pixels left and right of your background image)

#221F18 url(images/skins/yourskinsnamehere/misc/bg.jpg) repeat-y fixed center top