When a male acquires many nude pics, weather it be girlfriends or mistresses, they will find another male with a plethora of nude pics, and digitally trade with whatever they feel fit. This cycle grows to many other males, thus creating THE NUDE PIC EXCHANGE.
It's like Pokemon with a bunch of hot naked girls!
"Dude, where did you get a photo of my naked ex???"
A picture taken in a bra or underwear, sometimes completly naked and typically sent by someone looking for attention. People enjoy sending them and then when exposed, play dumb about it.