"scarlettjohanssoning" is a word that came up with the leaked picture of Scarlett Johansson. In the picture you can see Scarlett that took a picture of herself and her reflection in the mirror. The point is, she's actually naked on that picture.
So the word "scarlettjohanssoning" stands for someone taking a picture naked including the mirror reflection from the same angle.
A guy takes a picture in the same angle and also naked, uploads it and that is called scarlettjohanssoning.