1) Take a wall from CGtextures.
2) Each member adds their own graffiti to it.
3) Continue in this fashion until the wall is no longer usable.
4) Get new wall.
5) ???
6) ART!
Notes:
Please maintain the image quality. I've reduced the dimensions so it can be saved as a maximum quality jpeg without having a ridiculous filesize. Don't save it out of MSpaint.
Don't be selfish. Leave room. Don't completely cover other members' work.
Light profanity only. No pornography, nothing highly offensive. Have fun.