There are lots of murals, posters, graffiti, and other kinds of stuff on the walls in Oaxaca. Here's a random selection:
First, a great use of a crumbling wall.
IAGO, the graphics arts institute, has amazing murals on its walls.
Nice juxtaposition of prints.
Last, and definitely least, is this Chinese restaurant. The food was not very good (sadly, like all the Chinese food in Oaxaca) but it did have this bizarrely-leftover photo of Mao on the wall.