It's because it's a free image from Unsplash (http://unsplash.com/) which provides free hi-res photos. Highly recommend.
Its from http://unsplash.com/. We also have him on our landing page - gonna have to change it if he shows up everywhere :).
yeah usplash is a popular/great resource, and that unfortunately makes it almost useless. Right now there is another HN post on the front page that is using an image from that same MacBook Air series (as a background image): http://www.codingjohnson.com/an-awkward-conversation