If you don’t already know about these, I highly recommend Topher Gerkey’s Skins for the Skinless. They’re awesome, and Topher is one of the people who inspired me to try making my own Skins. They are Pay What You Want, but for Skins this good, the suggested price of $5 is a steal.
http://www.drivethrurpg.com/product/155325/Skins-for-the-Skinless
What’s the source for the art in this set of skins?
That I’m not sure of. You’d have to ask the creator.
Seth Abel from page 2 of the pdf: “All artwork was created by Topher Gerkey from original stock photography by a variety of artists – see each individual Skin for stock photography credits.”
On the first page of each Skin you can look in the lower left corner and see a stock photo credit, which is actually a clickable link taking you to the photographer’s profile. You can see the original unaltered stock photos there.