Angela, I occasionally use a photo in the Public Domain, so in my Notes to Reviewer I always include the URL from the actual photo and a brief note about it being Free Commercial Use (FCU).
I don't usually go into any explanation in my Artists' Notes, which are read by the buyer, unless I have used and altered an image by a well-known artist like Rembrandt.
The is no way to give credit to a Public Domain artist or photographer on the back of our cards.
Sometimes GCU artists will secretly embed a small reference to the contributing artist in their image itself.
Here's some more info that will help:
http://gcucommunity.com/2012/01/30/dash-of-inspiration-credit-where-credit-is-du
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