From the Snopes article
That was the day of King's Selma to Montgomery march so that is not Bernie Sanders in the image because he was not at that event (the actual say that it may be Paul Reese in the picture who actually was at the Selma march). No one is disputing that Sanders wa at King's March on Washington in 1963 just that is not a picture of it though. The Snopes article also says that Sanders' activism in the civil rights movement was "brief and localized".The above-displayed image was taken on 26 March 1965