Sure, but that was taken sometime later, as the two cars raced to the hospital. There was clearly an AR-15 on board, but I'm not aware of any photos or videos that show the SS vehicle in the first few seconds after the firing began, which could confirm or disprove Donahue's theory.
Donahue's theory was an attempt to account for the alignment of the entry/exit wounds in Kennedy's head, along with the type of injury, both of which were inconsistent with a shot from Oswald, but were consistent with a soft-pointed .223 coming from the SS car.
The freeway picture showing the AR-15 being held aloft was significant in light of the SS statements at the time to the effect that nobody in the security detail had or drew any weapons other than their service revolvers, which were loaded with .38 +P, would have made an entirely different kind of wound, and of which, none were fired that day anyway.
There is the famous photo by James Altgens that shows all the SS agents looking to their right, where Oswald's first (missed) shot hit the pavement behind them. Happens that it also shows the agent who picked up the AR-15 was the only one looking straight back at Oswald, just in time to see him line up his second (successful) shot.
Among the Dealy Plaza eyewitnesses, there was one Mr. Holland, standing on the overpass ahead of the motorcade, who said that he saw an agent "stand up in the President's car with a machinegun", only to fall down as if he had been shot. Definitely some errors in that, but you can guess what he must have seen.
But as with most such cases, you don't get to see the murderer plunging the knife into the victim's back in real time; you just have to piece together what had to have happened from the bits of information left behind. Donahue's theory accounts for those bits in a real-world way, and, as a special bonus, allows us to account for many others that most theorists ignore.
An example: Lots of people like to play the clip where Oswald tells the media, "I'm just a patsy!" Apparently he really thought so, but why? With Donahue's theory, you have a good guess: Oswald, while watching his target after his second shot, which he thinks may have connected, then sees Kennedy's head blown apart by a shot that he knows he did not fire. What would a guy like Oswald think in that situation?
Something like, "They set me up! Somebody was waiting for me to try this so they could blame it on me!", etc. Of course, he never got to elaborate on this theme, because of Jack Ruby, so we can't be sure that this is exactly what he meant. But all other explanations for the "patsy" either dismiss it outright, or bring in large, unwieldy and unrealistic conspiracies, which as I must insist, don't happen in real life like they do in movies. This was not the movies.