Its already possible to include CSS files that are not in the skin folder. In the CSS the image urls always must be relative to the CSS file itself. I would suggest just put a CSS file in the /Data/SiteImages folder and include that in your style.config and use that file to setup whatever background images you are using from that folder. Then your urls in the CSS file will just be url('imagename.png'); I would not recommend using image urls with http because then if you have a page using SSL it will cause a browser warning about the image not using https.
Best,
Joe