Of Nikolayev. The Rebbe's uncle, his father's brother.
COL asks "מי מכיר מי יודע" and a reader answered it was Reb Shmuel, brother of Reb Levik, the Rebbe's father. I remembered the pic in the SH and scanned it (see below.) Sorry about the quality, that was the best I could do under the circumstances. It is him! The pic was taken by the Soviet authorities and was discovered in his file that was just located.
Reb Shmuel was married to another daughter of Reb Meir Shlayme Yanovsky, sister of Rebbetzin Chana, the Rebbe's mother. Reb Shmuel hesitated, but The Rebbe Rashab OK'd the shidduch as long as they lived in separate cities. He was killed Al Kiddush Hashem by the Nazis during WWII.
In his prime.
(From Sefer HaTzetzo'im)