Normally if I need a reference photo, I go to Morguefile or Pixabay, or ask the photographer if they allow derivative works to be made from their photos. In short, I always have permission or use creative commons pics.
I came across 500px the other day, a site which seems to have many excellent photos uploaded by photographers. Despite looking around the site for almost and hour, I still can't find any info about USING the pics on there! Are they copyrighted? How do I buy them if so? There's no way to add them to a cart like stock photos... Anybody ever use this site or have any idea how the photos are licensed for use?