From the photo page: This work is dedicated to the Public Domain. For non-US use it is also placed under a Creative Commons CC0 designation.

Sullivan has stated: "You may use this image for any purpose, including commercial. As the owner I have explicitly placed it in the public domain. If you do use it, please consider linking back to pdphoto.org. Use it for whatever you want. Use it all. Sell it. Mangle it. Whatever you want. Please. But it would be polite to let me know so I can at least enjoy whatever creative use you're putting it to. This doesn't mean that you can take the material and then copyright it yourself. It’s in the public domain and that’s where I want it to stay."

Attention: not all pictures from pdphoto.org are in public domain! Please check the metadata on the source before applying this license tag.