Pollice Verso by Jean-Leon Gerome, 19th century. Ridley Scott was inspired by this painting to direct Gladiator. But Roman authorities actually used a thumbs UP signal to indicate that the loser of the present battle should die.
"To teaching friends: I use this in high school as a formative assessment before moving on. I don't have a poster or anything, but it really helps you get a grasp on where the class stands on a tough concept." I might make a poster!!!