I'll just chime in to suggest that perhaps the image is an artist's rendition, and the artist had never seen a Medusa quite that clearly (given that he is not petrified).

Or maybe at a distance of 30 ft. or more they have a non-(ex), non-(sp), and non-(su) effect that causes them to look totally human to all observers.

Or they just usually wear much more clothing and hide their snakes under hoods?