English: Rabbi en:Menachem Mendel Schneersohn, the Tzemach Tzedek, the Third Rabbi of Lubavitch. The seventh Lubavitcher Rebbe was named similarly: en:Menachem Mendel Schneerson (without an "h" in his surname.)
The portrait was painted by an unknown non-Jewish artist, towards the end of the Tzemach Tzedek's life; the exact year is not known. All published pictures of the Tzemach Tzedek are derived from this one. The original portrait was in the possession of the Tzemach Tzedek's grandson Rabbi Shlomo Zalman Schneersohn (the Magen Avos), who was rebbe in Kopust from 1866 to 1900. As of 1991 it was in the possession of the Magen Avos's great-grandson, Yehuda Leib Ginsburg, of Moscow. A copy is in the Chabad Library, and a detailed discussion (in Hebrew) of its authenticity is found on pages 51-56 of PDF document this.