I have to say that the idea is cool, but you could do so much better and cheaper doing it yourself, I mean, Space Invader graphics have to be the simplest to copy/trace! I bought this projector at a craft store (Michaels here in the Chicago Burbs) that project an image on the wall, like an overhead projector, for like $40-$50 bucks. Seems like a lot, but I can use it over, and over, etc.. I scanned some Dr. Seuss images from some books and then projected them on the wall for one of my daughters bedroom. We just traced with black paint pens and then colored them in with the proper color paint. It was very simple and the results were professional looking. Spend the money on something like this and then you could project any number of images on the walls to paint. I am even considering doing this for the sides of my cab, but it's black and I'm not sure how well it would show up for tracing....
Anyway, good idea.....