I don’t own any right for the pictures I post on my blog so I guess you’re not asking the right person. All my Glenn Gould and Leonard Bernstein photos are reblogged posts from other blogs. Maybe you should try to ask the runners of these blogs where they got their pictures from (the source of a pic usually appears with the post).
You’re working on a beautiful project and I’m sorry I can’t help you more. Pictures on tumblr often come from official websites like http://life.time.com/ for instance. I think you must ask them if you want to use any of their photos.
I hope my answer helped you, even if in a small proportion !
Best regards !
P.S: If any of Fantômas’s followers has clearer and more precise informations, feel free to leave a message ;-) !