The first user posts a screencap from a movie or a tv-show, and then it's up to the rest to try and guess which one it is. The first person to get it right gets to post a new screencap, and around it goes...
Fairly simple right? Well, it depends on how simple you want it to be (posting a screencap from Prison Break might not be very hard, but a screencap from 'The Puppet Films of Jirí Trnka' might be a bit too hard

And to keep things going: If it's too hard, then the person that posted the screen can give clues. And if a person guessed right but hasn't posted a new screen in a few days (let's say 3?), then anyone can put up a new one.
I'll get things going with a fairly easy one:
