Hermes (the messenger god and the god of merchants and theives) with a huge phallus. You recognize Hermes by the small wings on his feet and the kerykeion (heralds staff).
It is important to remember that erotic/comic paintings were common in the ancient world. I must unfortunately admit that I do not know where this painting originate from (except that it is from the Campanian region, probably Pompeii).
We believe that these images were intended to protect people from the evil eye. The point was to avoid envious looks (sight was perceived as some sort of laser beams that would affect the person who got hit by them) and this was achieved by making people laugh, preferably by exposing them to a picture depicting a huge erected penis.