One of the things I've seen done that came from Dr. Faust's Painting CLinic on YouTube is to print what you want as the grafitti out on an ink jet after mirroring the image. Cut the image out as cleanly as you can, then you give it a layer of modpodge (If it has a lot of white in the image, use the white drying version, if not, use the clear), then press it firmly onto the model where you want it and let it dry entirely.
Then use a small sponge and work at the printer paper with water. Go slowly and let it dry often, the paper comes off and leaves the ink behind and you need to seal it.
I believe that it's demonstrated in video.