Painting your car isn't actually a bad idea. Yes, I fully support the concept of drawing on your car. Maybe even loading images into the game and using them as stickers.
but would it be possible without lagging the game? i would like that too but it doesn't seem like it would work
it would lag as much as Microsoft paint. Graffiti would just be a drawing application, and a little texture overlay on your car that would lag as much as any texture -- not at all.
I think only having some image placing as stickers system is sufficient. It seems easier to achieve rather than making a whole "free painting"-system