As we saw in "Creatively Defaced Textbooks," it's easy enough to create drawings in a book that you take home with you, or hide behind the back of the student in front of you. It's a much greater challenge to deface—or upgrade, depending on your point of view—a streetscape, where your artistic talents may draw the unwanted attention of the authorities.
But, you know, nothing stops art.
Sadly there's no attribution as to the source of these photos, but Memolition has compiled a collection of nearly 50 shots of street art, some so large-scale they must have been sanctioned, others clearly done while the city slept. And as with the textbook shots, they appear to be globally sourced.
All are SFW; check out the rest here.