Trotsky (fourth from left) and Stalin (in white) carrying the coffin of Felix Dzerzhinsky in Moscow during July 1926. Footage of this scene can be viewed here, other photographs of the procession where both men are visible can be viewed here and here. Kudos to this forum for helping me locate these images.
Trotsky wrote the following in 1937 (four years after the Holocaust began—according to the United States Holocaust Memorial Museum's definition of the Holocaust):
"The memory of mankind is magnanimous as regards the application of harsh measures in the service of great historical goals. But history will not pardon a single drop of blood shed in sacrifice to the new Moloch of self-will and privilege. Moral sensibility finds its highest satisfaction in the immutable conviction that historical retribution will correspond to the scope of the crime. Revolution will unlock all the secret compartments, review all the trials, rehabilitate the slandered, raise memorials to the victims of the wantonness and cover with eternal infamy the names of the executioners. Stalin will depart from the scene laden with all the crimes which he has committed—not only as the gravedigger of the revolution but as the most sinister figure in the history of mankind."
- Leon Trotsky, 'The Beginning of the End,' Socialist Appeal (New York), October 16, 1937, p.3.