I understand it from the other party, since this might be his/her way of making a living: selling photographs. I know I wouldn't like to see my pictures floating around on the internet without at least giving me credit for it.
Ofcourse these things happen and like you, I've always collected many pictures in the earlier days and would never be able to find the source of those. I decided not to use them though and just use my own images or google for a good one, so I can link back to the source.
I think you handeled it pretty nicely. You've pointed them to a place where they can earn more for their photos and if you introduce them, you'll give them a nice flying start here!
Well I can just hope it wouldn't be blown out of proportion!