there should be a new option for pixmap object of the theme that allowed to mark it as the background image, while still providing a default choice. When a theme with a defined background is selected, gdmsetup should have the option to selecta a background image that were lodaded into the pixmap marked as such. Most of the time, the only reason to not to choose a theme is its background, and there are lots of identical themes with the only difference being the background image, so it seems like it would be a very popular feature for 2.18 =D.