I can't vouch for your images. In mine, the line that might be a rifle was cut off at about the length of a rifle, not full vertical but closer to vertical than horizontal. And that's the impression I had or I wouldn't have pursued this in the first place.
Okay, you're looking at a different line. One that appears to be immediately behind him, I think? I was thinking of the longer line to the left.
In any case, Hickey said he picked the rifle up from the floor.
From his account: "At the end of the last report I reached to the bottom of the car and picked up the AR 15 rifle, cocked and loaded it, and turned to the rear. At this point the cars were passing under the over-pass and as a result we had left the scene of the shooting. I kept the AR 15 rifle ready as we proceeded at a high rate of speed to the hospital."
Here he is with it raised (but not turned to the rear at all) as they leave the underpass:
Whatever the case it looks to me that he's not standing up at the time of the head shot.