I find the poster offensive...not for political reasons, but for logical ones. What the hell does socialism have to do with the "Dark Knight's" Joker? He was for anarchy, chaos, and breaking the rules...sounds closer to a libertarian view then a socialist view. I guess you could say his programs are like the social experiments like the hospital and the boat, but that's a stretch.
That Joker was against society. He was for chaos, death, destruction, and lawlessness. While that fits one definition of anarchy, I don't see why anyone that calls himself an anarchist as a moral position because it allows for positive social interaction would want to associate himself with that character. It seems to me that associating the Joker with anarchy creates a false dichotomy that mis-characterizes the issue.
The statists who constantly seek to socially engineer the world actually create disorder, chaos, and destruction. I think comparing the Joker to them is more apt than calling him an "anarchist".