At the beginning of episode 5, right after you've completed Flashback 5, Kate let's you out of the cell. Once she does, walk over to where the lava lamp is. To the right of the lava lamp is a bookcase; and, on the roof above the right hand side of the bookcase you will find blood stains. Take a photo of them to get your achievement.
The video will show you exactly (I did not make nor do I take credit for this video. Video was created by YouTube user SaggDog7):