Creative Speculation

> why not show the next few frames where the flap of skull and hair is blown back and the brains out the back of the car...

Because it's irrelevant to the issue, so let's try this again: The first frame is the last one before the hit, and the second frame is the first one after the hit. The bullet only imparted momentum while it was in contact with the head, between those two frames. Momentum transfer was simultaneous with the bullet slowing down while passing through the head. The second frame shows the immediate result of that momentum: an unmistakeable, violent forward snap, which proves the shot came from behind.

The frame after that one just shows the head returning to approximately the starting position -- it looks as if his chin bounces off his chest -- and the frame after that shows the start of the "back and to the left" motion, which starts slowly and then accelerates. That motion simply CANNOT be caused by the momentum of the bullet, because the bullet is long gone from the scene by that frame. This is "basic science," which you implied plays some part in your beliefs.

> if you can watch it and tell me the head shot did not send the head back I will give up completely even trying to discuss this because it will be impossible to convince you that your eyes are not lying to you...

Again, my gif shows very clearly the immediate result of the hit. As a matter of fact, I discovered the head snap for myself after someone made a similar challenge to watch the thing closely, which I did by watching frame by frame. But then looking around the web, I found it that the forward head snap is fairly common knowledge among people who find the JFK conspiricists' evidence and arguments unconvincing, and some people have measured it very precisely to be about 2.5 inches.

After accusing people who disagree with you of just being too fearful to face the truth, and after claiming that "basic science" is your reason, it would behoove you to watch that gif a few more times and don't be afraid to give it some real thought.

(ETA: I also meant to correct your misconception that the head was already traveling forward. Examining the individual frames shows that it most definitely was not. It's in the same place for many frames, and then in the frame immediately after the hit it's been pushed forward very violently, as would be expected by a shot from the rear.)