Here is an example of what the original poster is referring to:
http://www.istockphoto.com/stock-photo-9389492-ox-cart.phpNot part of the Hulton archives, as far as I can see. How
would that work? If an image is public domain, anyone can use it. I have a feeling an exclusive can upload and sell it, but you will notice in the above photographers description, she mentions it is part of public domain. That way, the buyer knows it is not an exclusive image, merely being sold by an exclusive.
I suppose that means that there could be the same image in someone else's portfolio? It will be interesting to hear how that works.