I asked Mary Moorman, and she said she recalls them happening simultaneously: the fatal head shot and her snapping the picture.
I'm sure she's being honest, and I don't doubt that she remembers it correctly. But remember that we are talking about a tiny fraction of a second difference. If the fatal head shot occurred at Z-313, and she took her picture at Z-316, as I suspect, that difference is only 3/18 of a second, which is to say 1/6 of a second. In the midst of everything that was going on, that could easily be perceived as simultaneous.