The USHMM explains how the photographs came into the public domain:
According to the Zentrale Stelle in Germany (Zst. II 204 AR 1218/70), these Jews were collected by the German Gendarmerie and Ukrainian Schutzmannschaft during the liquidation of the Mizocz ghetto, which held roughly 1,700 Jews. On the eve of the ghetto's liquidation (13 October 1942), some of the inhabitants rose up against the Germans and were defeated after a short battle. The remaining members of the community were transported from the ghetto to this ravine in the Sdolbunov Gebietskommissariat, south of Rovno, where they were executed. Information regarding this action, including the photos, were acquired from a man named Hille, who was the Bezirks-Oberwachtmeister of the Gendarmerie at the time. Hille apparently gave the five photos (there were originally seven) to the company lawyer of a textile firm in Kunert, Czechoslovakia, where he worked as a doorman after the war. The Czech government confiscated the photos from the lawyer in 1946 and they subsequently became public. That the photos indeed show the shooting of Jews in connection with the liquidation of the ghetto was also confirmed by a statement of Gendarmerie-Gebietsfuehrer Josef Paur in 1961.Confirmation of the authenticity of the photos was thus provided by a perpetrator, Josef Paur, whose trial can be found here.
The ignorance of deniers relating to these photos can be gauged from this Cesspit thread and this article by Porter. Neither of them acknowledges the trial of Paur nor the fact that the perpetrators were Gendarmerie and Ukrainians (hence the failure of the deniers to identify the uniforms). Note also how Porter criticizes one photograph because it contains blood and another photograph because blood is supposedly absent!