Mindfucks are pics where it may seem like a normal pic of someone or something, but there's something creepy or fucked up that's not too obvious at first viewing.
For ex., the pic below may seem like a normal girl posing to show her boobs n thong n stuff, which is what you'd notice first. But the mindfuck is when you notice the reflection of a wierd, creepy face, that makes you shit bricks.