Totally thought it was Connally at first but Gerda's work suggests to me that Jackie is pushing JFK's body up. Maybe some kind of shock/trauma thing.
Sorry Dan that's not Kennedy. Jackie's crouching and looking downward, most probably holding Kennedy's hair on (like she said she did). There's no pink arms on the figure propping it up like you think she's doing. Her pink hat is in front of that body which means she's in the back seat and Connolly is in the background further away from her. There's also no flap of scalp on the head of that figure as well.
The more interesting thing, of course, is why Connolly is sitting up? He'd just had his lung collapsed and other painful injuries. Earlier in the Z film, you can see him falling toward back first to his wife. So for some reason he sat up momentarily - perhaps because of shock or panic?