Not sure what you mean by "graffiti" in terms of photo effects. But on photobooth you have a lot of special effects which you can choose from before you take the shot. Failing that, you can always modify and add effects in iPhoto.
I wouldn't bother, mate. The ride will be like sitting on concrete and every speed bump will grind your bodywork. There are other ways to individualise your car.
I think it should be obvious to you when someone is flirting. What might not be obvious is their motive and intentions. Be careful. People are not always what they seem and seldom what they say they are.