By DEREK GATOPOULOS and PETROS GIANNAKOURIS ATHENS, Greece — Thrasivoulos Marakis grew up hearing stories about the grandfather he was named after but who he never met — about how the tall man was executed during Nazi reprisals in Greece during World War II. For decades, the only image Marakis had of his grandfather came from a worn family portrait picture. But last month another photograph emerged. An online auction contained a photograph showing his grandfather walking calmly toward a firing squad alongside other prisoners. The image shook the Marakis family and has stirred powerful emotions across Greece, where the...