Find or take image of graffiti art.
Paste into the image of girls as a new layer.
Add a black layer mask to hide the graffiti, then paint on mask with a white brush to reveal it in the proper location.
Shape the graffiti to match the texture of the wall with the displace filter.
Change the layer blending mode to allow the wall texture to show. I used Overlay with opacity of 80%
Blur the graffiti slightly to simulate the distance behind the subjects.

Forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=32448500

original image by Portraits by Lorna