tl;dr: They’re all mine, and before today were mostly all on my Flickr. I selected, obviously, for pics that might look interesting when cropped to the window available above. WordPress randomly selects one when the page is built, so you get a different one nearly every time.
This link is to a gallery of the full-size images, in the event you’re curious to see them, or their context. The oldest of them are from 2005, and were taken with a point-and-shoot.