How did Indian engineer Shanmuga spot Vikram Lander by comparing pics?

NASA has credited Indian engineer Shanmuga Subramanian for discovering Chandrayaan 2's Vikram Lander on the Moon by comparing pictures taken before and after the scheduled landing date. Subramanian compared a picture taken in December 2017 and another taken on September 17. He found a white dot in the September 17 picture, which he said wasn't there in the old picture.