Selected collections available online through the New York Public Library (NYPL)
From the USHMM: "The Photo Archives is a rich and expanding collection of photographic images gathered from private donors, archives, libraries, museums, and photo agencies from around the world. The bulk of the collection spans the period from the end of World War I to the early 1950s."
From Yad Vashem: "Yad Vashem houses the largest collections of photographs relating to the Holocaust in the world. These photographs, which present people, places and events in visual form, are of immense historical, educational and commemorative value. Two central Yad Vashem photograph collections are presented on the Yad Vashem website: The Yad Vashem Archive collection of historical photos and the Yad Vashem Hall of Names collection of photographs."