The code word was “chill.” That’s what the crew with Shepard Fairey, the cult graphic artist known for his screen prints and stickers of the wrestler Andre the Giant, had been instructed to say if a police car rolled by as Mr. Fairey was wheat-pasting one recent night here, illegally tagging warehouse walls and empty billboards with his black-and-white images. Then Mr. Fairey and his helpers would know to make a run for it, to avoid yet another arrest.