This street art by Jesse Olwen is on a wall, but it's not graffiti. Oh, it looks like graffiti, but it is attached to a wall and can be removed and displayed in a gallery without defacing the property. But while it's outside, it attaches a pleasant picture to a ruin.
I make paintings on wood (which I cut into a graffiti-ish shape. In this piece the words read "PUNK NOT DEAD"), then I paint imagery of things which I consider to be a nice to contrast to the the location I place it in. This piece shows a couple of kittens in front of roses hanging over a demolished pit a few blocks from my apartment here in Incheon South Korea. These are two 4'x4' pieces together, acrylic on plywood.
See more of Olwen's artwork, both inside and outside, at his site. Link