My girlfriend came back from visiting her grandparents and according to her they own a set of negatives that were taken by a family friend for intelligence purposes who later passed them to her great grandfather for keeping. I doubt the family would ever want to donate them or sell them to a museum at a low price due to their lower socioeconomic background.
What type of value do pictures like these have today to historians?
This is fascinating. Do you have more details on how they came to be in her possession? The relationship between the photographer and Mussolini? What kind of "intelligence" were they involved in? What kind of condition are they in? A full story would really help.
Even without the originals, with some idea of provenance (the story of how the photographs were taken and the negatives passed), a short term loan to a museum for a high quality scan would be useful. If you were planning to do this, find one that has interest in the period. If you state which country you are in, that may help people make suggestions.