Before every game, a small group of people are able to finagle field passes to enjoy pre-game warm-ups from the sideline. If you get one of these passes, you can take pictures down near the field, and get a chance to see just how big these guys are. If you're on Twitter, Jed York often will give out field passes to each game through some kind of contest.
The guy in the above picture (Scot Schaffer) was down on the field with his son when they decided to take a selfie near the end zone. It just so happened San Francisco 49ers defensive tackle Tony Jerod-Eddie was taking part in warmups right nearby. He noticed the father and son taking the picture, and decided to insert himself in it. The result is this fantastic photo bomb.