Update: This article is about an older design of this website and it's no longer relevant.
Almost all header images (in this case they call them "Hero images") on articles pages, like the one on this page, came from Unsplash.
The reason for that is CC0 license. CC0 license allows you to do anything you want with an image: use it for any kind of work, edit it if you want, republish it, even sell it, without any permission or attribution. Even attribution is not required, I always include link to Unsplash page of the author at the end of every article here. If you don't see that link, that means that I took the picture.