1. (n.) An involuntary reflex that occurs when you get the third photo of someone whom you believed was attractive, but looks a lot worse in this one. Everyone has one or two great photos that make them look a lot better than they really are. When this person lets down their guard and sends a more natural photo, the illusion of attractiveness is shattered, and the wince occurs.
I was writing to this hot girl who gave me a glamour shot and a downshirt. Then, she sent me a picture of her sitting on her couch in sweats with the cat. That made me do a third-pic wince and cut off all contact with her.