Once upon a time a man by the name of Luis Walter Alvarez saw some still frames from the Zapruder film published in Life Magazine. He was intrigued by the motion of the President's head from the fatal shot. Being that Alvarez was both an optics and photoanalysis expert, he decided to apply the scientific method and discovered that the motion of Kennedy's head is entirely consistent with the shot having come from the rear.
Who, you might ask, is Luis Walter Alvarez and why should we take his word about this? Alvarez won the 1968 Nobel Prize in physics. He won the 1961 Albert Einstein Award in physics. He holds over 40 patents. He invented Ground Control Radar, for which he won the Collier Trophy from the National Aeronautics Association. Some of his findings were crucial for the development of the hydrogen bomb. In other words, he was a REALLY smart guy in the field of physics. So excuse me if I will take his finding over those of anonymous internet scribblers.