These are Whispers, which is an app that lets you post anonymously but all posts must have a background photo (that I think you choose from the Whisper database), which is why there's a ton of random memes with random photos behind them. Also, Dexter-related memes are currently hot, and that's Dexter himself, so it's not quite so random.

EDIT: These may have come from Pinterest as well, I've seen a lot on there like that when looking them up