Fixed/Scrolling Background Image for NOVA 2.4

Fixed/Scrolling Background Image for NOVA 2.4

I've browsed through the previous entries in the skinning section, and come across this for NOVA1. However, I'd love todo this with NOVA2.

I'm seeking to try and put an image behind the pages on the site, whether its fixed or scrolls I don't mind. Just something to add to the graphics and whatnot. Which files, and what coding I'd need to input, are my main questions.

I'm not sure if it'll be different for each skin, but any help would be greatly appreciated!

(I should also note, I'd be doing this on a self-modded version of LadyBird's Shadow skin, which is based on the Lightness skin.)