So had a discussion with Patty about the difficulties of being an umpire and trying to be in the right place at the right time. First picture is the ball coming to home plate, runner sliding in. Remember that he has to get to the bag before the glove of the catcher touches him with the ball. Judging by the position of all three in this instant, you make the call.
The second image tells you what the umpire saw, and shows you where he saw it from. I do not think he has the best angle view, but where else could he be to make the call? No wonder he has the toughest job in Baseball.