It looks like the reflection of light off the head of the man sitting in the front passengers seat. For those who doubt me, I have one word for you - Brylcreem. Google it. That's how a man's hair can reflect the light of the sun so very well. This was the style back then. The driver didn't shoot Kennedy.
Download your own "Driver Shoots Kennedy" in flash by right clicking and choosing save. Here's another "Driver Shoots Kennedy." This one has a nice decorative wood frame. Go ahead, I've got lots of them. Share them with your friends and family. Give a "Driver Shoots Kennedy" to your mother in law for her birthday.