Some years ago, a talented photographer by the name of Carol Highsmith traveled the world taking lovely photographs along the way. Photos she could have charged a pretty penny for, but which she instead generously donated to the Library of Congress. Her photos are now free for anybody to use as they are in the public domain.
However in an infuriating turn of events, Getty Images copied her entire collection over to their own archives, then when she used one of her own photos on an article she published, they had the audacity to send her a bill for it. For her own picture! One she donated with the express intent that anybody should be able to use it for free!
She successfully sued Getty images for 1 billion, so there's a happy ending to this story. But let's further celebrate her talent by making use of her body of work in the way she intended: Freely, available to all, and without fear that some bully like Getty will come knocking on our doors, looking for a cut of our earnings.
The legality of how images are used on this site is still unclear to me. It seems like it falls under fair use, but then the fact that we often make money from our posts complicates the equation somewhat. By preferentially using these free images, that uncertainty can be circumvented. Besides which, they're just damned good photographs.
Without flapping my jaw any more, here's the full archive of photographs for your enjoyment and free use.