The image is by Matt Wang, who combined a photograph of Los Angeles with an image from the Rosetta spacecraft rendezvousing with Churyumov-Gerasimenko, prior to going into orbit and eventually landing.
While Rosetta will be doing its best to touch down very gently on the comet’s surface, we can’t count on Churyumov-Gerasimenko doing the same if it ever paid LA a visit. You can get some idea of what would happen in such a collision here, although the scale of the damage depends greatly on the impact speed you choose to enter.
