Years ago (we're talking decades) I ran into a small program that randomly generated raytraced images (think transparent orbs, lens flares, reflection etc), suitable for saving as wallpapers. It was a C/C++ program that ran on Linux. I've long since lost the name and the source code, and I wonder if there's anything like that out there now?
POV-Ray, perhaps? You give it a scene description text file and it will render a raytraced image of the scene for you. You'd need to find or write an appropriate scene description though for what you want to randomize.