I'm not a tax expert, but this is the experience I had. Two years ago I donated my ancestor's shawl to the D.A.R. Museum in Washington D.C. Since they are a non-profit institution, I was able to use the estimated value of my donation as a right-off on my taxes at the end of that year. The DAR Museum was prohibited from appraising the value of the shawl, but what the textiles conservator I worked with suggested was to find similar 18th century silk printed shawls online, add an additional 20% for the provenance of it belonging to my ancestor (she was one of the few women recognized for her service in the American Revolution) and subtract 5% for the condition (slight deterioration after 200+ years). The museum had a form that I filled out surrendering the shawl to them and I got a copy, which I took to my tax guy along with the value estimate.
For me, the real value was in knowing that my ancestor's shawl would have the absolute best care and protection possible, in a museum that was devoted solely to the American Revolution. I also knew that it would provide a central place where my ancestor's descendants could come to see it (along with her wedding dress which was donated by my great-grandmother).
It was an extremely difficult decision to part with it, but one I feel very good about. I can still go see it any time I want and the conservator there is wonderful. When I went to see the wedding dress, she spent well over an hour with me going over all the details of the dress, telling me what it meant about my ancestor's status, showing me how the treasured dress had been altered for her daughters later. There is really something to be said for placing unique pieces of history into the hands of the people and organizations who can offer that kind of insight into an era in history and making them available to others for study.
Nothing conveys history like the real thing that was really there. I hope that your photo negatives can touch people's souls in a way that will insure that nothing like the Holocaust ever happens again. I commend you on the decision to donate them too. I know how hard that is to do.