Medusa

This is painted not on a flat canvas but on a round wooden shield covered in canvas, a ceremonial parade buckler of the kind meant to be shown rather than fought with. It was made around 1597 as a gift from Caravaggio's patron, Cardinal Del Monte, to the Grand Duke of Tuscany, who kept a famous armoury in Florence. The subject fits the object exactly. In the myth Medusa turned men to stone, and Perseus beat her by watching her reflection in a polished shield rather than looking straight at her. So Caravaggio painted the severed head onto the very kind of surface that defeated her. The face is caught in the instant of the killing blow, eyes wide, mouth open, blood still coming from the neck, the snakes on her scalp still writhing. The curve of the shield makes the head seem to bulge out toward you.