photoshop layers :) i'll try to explain it.... open two pictures, the one that you want to have a space overlay with, and another of whatever picture (galaxy or not). drag the galaxy image into the "layers" on the right side of photoshop (CS4... idk if it's different for older versions) and make sure the layer is on top of your original image. then above the layers, there should be a dropdown menu with different effects for the layers. i usually use dark light or overlay... just play around with all of them to get the right lighting you'd like :) it's fun with other images too!