What criteria are required for a photo to be labeled as "Premium"?

When I search for photos it labels some photos as "Premium" and obviously they are more expensive. I noticed I could find similar photos with the same qualities (to my eyes) that are not marked as Premium. So I wonder what makes a photo be considered Premium? 

Thank you

Note:  I posted this question here mistakenly and no one responded. I don't know how to delete it. I don't want to be considered a spammer.