I've been thinking about how to do this as a future project -- I have some anime/cartoon themes I'd like on the back wall, painted up high. Maybe even the back of the garage door, since that's an unused space.
My idea is to find the image I want (Say, of Transformers Generation 1 WheelJack or some InitialD image) and shine it up on the surface with a projector, trace it out with a pencil and then paint it with hobby paint (or even mask and spray large areas). Since I'm interested in cartoons, I don't really have to worry about smooth shading. Something like this:
or
I could even do an edge enhance in GIMP or something to make the tracing easier.