Hi Peter (RE message above),
A few points in response - first, I am not a pro photographer and view my images as of some minor cultural but no commercial value - they are really little more than snapshots, most of not tremendously high resolution, of interest to a pretty boutique, and cash poor, market... if I were trying to sell my pictures for a living things would obviously be different (and they would not be of folk singers :) )
Second, Google images is your friend! It shows the world where the original of your image is and who has copied it, either with or without authorization, and with or without credit.
Third, Wikipedia is the most cited site in the world (at or near the top in web results for any search) and is scrupulous about crediting the sources of its images. Click on the image and the details are all there, including the date of original upload, the creator, and licence conditions if any, all good for legal purposes if ever needed.
Letting an image into the world without constraints is to my mind a bit like writing a song. If you let others hear it, if it is any good it can take on a life of its own and others might sing it. Some may lose sight of the source (accidentally or deliberately) but hey, there are people - many on this forum - who enjoy chasing such things down. I honestly think that finding the true source of an image for credit purposes is a no brainer these days with this interweb thingy.
And if someone can get rich re-selling my 30-40 images of a few folkies, that are already available for free to anyone who wants them, then I wish them good luck.
Just my 2 cents worth of course.
Best - Tony