All gaming graphics, including GUI background images are stored in one large file called OP2_ART.BMP. To this date, we don't know exactly how to access those images inside OP2_ART.BMP, but i know op2hacker and hooman are (or have been) working on it. I, myself might also start on it, given the time, and whatever is known now...
After we've figured out how it works, the simplest way (not the easiest) is to extract ALL images to separate BMPs, and then also create a tool to re-compile them back into OP2_ART.BMP. This way any artist can edit the images one by one. I believe OP2_ART.BMP contains hundreds of images, so i'd reserve some time for that if i were you !