For his show, “Crude Oils,” Banksy stocked a Notting Hill gallery with two hundred free-roaming rats. Rodents are a favorite motif. “Like most people, I have a fantasy that all the little powerless losers will gang up together,” Banksy wrote in “Existencilism.” “That all the vermin will get some good equipment and then the underground will go overground and tear this city apart.” His most famous street paintings are a series of black-and-white stencilled rats, the majority of them slightly larger than life-size. Each is different, but they all possess an impish poignancy that made them an immediate hit with London pedestrians. One, a “gangster rat,” painted on a wall near the Smithfield market, wears a peace-sign medallion and carries a sign that says “Welcome to Hell.” Another pleads, “Please love me.”