Tweek likes to sneak little photos and videos of himself and Craig while they're having sex and post it to social media (probably Snapchat) for all their friends to see, especially when Craig is fucking him from behind and all you can see is part of Tweek's face and Craig's torso...but there's no mistaking who it is and what they're doing. Craig usually doesn't realize he's being recorded until after Tweek's posted it.