Adding a background image for Omega subtheme
You shouldn't need to do any of the preprocess stuff.  We recently had some problems getting things to apply via the global.css file - but we figured them out.

Here is the trick of it.  Even though you apply X in the global.css file, if Y is applied in one of the stock css files, and you don't turn off Y (via css in your global.css) - and Y in some way impacts X - X won't be active.

I recommend either Firefox and Firebug - or Chrome and "right click -> inspect" - and really scrutinize the DOM.

Also - regarding your image file - you might try specifying a full path.  We have had issues with relative paths.

I hope this helps.