All photos sent out in the monthly free email newsletter and stored in the Pro folder are published under the Creative Commons Zero license
"You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission." Websites, ebook covers, presentations...you name it, you can use it!
You can't, however, commit fraud by claiming the work as your own (e.g. submit the original photos in a contest under your name).
It would be great if, where you can, you could let others know where you got the photo by linking back to this site!