For the sake of a prominent researcher who inquired with me about it, I am going to lay out the multi-pronged paradigm of why Jack Ruby was not the Garage Shooter of Lee Harvey Oswald.
It starts with the images of the Garage Shooter. If you compare him to images of Jack Ruby, including images of Ruby on 11/24/63, you realize that they were different men.
1. The Garage Shooter was shorter than Ruby, who was 5'9", which is average height, then and now. There were about 50 men in that garage, and the shortest one, that we can see, is the Garage Shooter. He was well shorter than 5'9". He was shorter than Captain Will Fritz, who was a short man, certainly no taller than 5'7".
2. Besides being short in height, the Garage Shooter had a very short neck. He had a thick, short, burly neck. Ruby, in comparison, had a longer thinner neck.
3. And, the above image shows us how different the hair was between the Shooter and Ruby. The Shooter on the left was wearing a toupee'. You notice that you don't see any hair growing out of his head. That is a rug. And notice that the hair ends abruptly, that there is no tapering and no scruffy hair growth on his neck. Compare to Ruby's mug shot on the right.
There's more that I could say about their conflicting physicality, but I want to keep this concise.
4. Next, we'll compare the clothes. Ruby wore black socks and high-top black work shoes. The Shooter wore light socks. We don't know the color, since it's a black and white photo, but they were definitely light. And his shoes were dressy wingtips.
So, it's the Shooter on the left and Ruby in his mug shot on the right. The authorities did become aware of this discrepancy, and in response to it, they made the claim that Ruby's shoes and socks were replaced by the Dallas Police. But, that is ridiculous because it was a City Jail, and they didn't normally provide clothes to anybody. Did they provide any clothes to Oswald? No. Even the clothes he wore on Sunday morning were his own clothes. It is simply preposterous to claim that the Dallas Police provided shirt, pants, shoes, and socks to an overnight detainee. So, that claim was a desperate and pathetic lie.
There is only one image of the Garage Shooter's face, and it was included in the NBC 30 year retrospective on the Oswald shooting, which was broadcast in 1963. It clearly shows the the round-faced Shooter was not Jack Ruby. 
5. Ruby had no motive, no intention, and no memory of shooting Oswald. The way he related what happened is that he went down to the garage, and then police started jumping him, and he did not know why. And as they were pouncing on him, he asked them why because he had no idea why. Then, they dragged him up to the 5th floor, and that is where they told him that he shot Oswald. It was a complete shock to Ruby. It was as though he briefly entered a deranged mental state, and that is what his lawyer, Melvin Belli argued in court, that Ruby had a form of epilepsy, and that movements in shooting Oswald were part of an epileptic seizure. It is ridiculous, not only because Ruby did not have epilepsy, but because it is impossible for the spontaneous and unorganized neurological discharges involved in a seizure to manifest as someone pulling the trigger of a gun.
5. Ruby had no motive, no intention, and no memory of shooting Oswald. The way he related what happened is that he went down to the garage, and then police started jumping him, and he did not know why. And as they were pouncing on him, he asked them why because he had no idea why. Then, they dragged him up to the 5th floor, and that is where they told him that he shot Oswald. It was a complete shock to Ruby. It was as though he briefly entered a deranged mental state, and that is what his lawyer, Melvin Belli argued in court, that Ruby had a form of epilepsy, and that movements in shooting Oswald were part of an epileptic seizure. It is ridiculous, not only because Ruby did not have epilepsy, but because it is impossible for the spontaneous and unorganized neurological discharges involved in a seizure to manifest as someone pulling the trigger of a gun.
So, why did Belli resort to this? It's because of what Ruby told him, which was that he had intention and no thought whatsoever of shooting Oswald, and he had no memory of doing it. So, rather than considering that the reason Ruby couldn't remember shooting Oswald was because he didn't shoot him, Belli resorted to flights of fancy. Why? Because, like most people, Belli had the disease of Americana, which is the belief in America, that it is a good country, and that the killings of JFK and Oswald could not have been done by those wielding authority.
6. Both the Beers and Jackson photos show the Garage Shooter pulling the trigger with his middle finger. Now, why would Jack Ruby do that? Everybody, including very young boys with play guns, know that the trigger finger is the index finger. There was nothing defective about Ruby's right hand, but ironically, his left index finger was partially amputated.
If you look closely, you'll see that Ruby's left index finger was partially amputated. But, it had no bearing on how he would shoot a gun because he was right-handed. Most likely, what happened is that the people who planned this spectacle were confused. They mistakenly thought that Ruby's right hand was impaired, and they assumed that the middle-finger shooting would fit him. But, they wre misinformed. It's what's called a production error.
So, in this posed photo, James Bookhout (who played the role of Ruby in the garage) used his middle finger.
So, that is the Jackson photo showing the middle finger on the trigger. But, now that you know that Ruby had nothing wrong with his right hand, you know he wouldn't have done that. And when his lawyers asked him why he did it, his response was, "I don't even remember shooting Oswald at all, so how can I tell you why I used my middle finger?" That should have alerted his lawyers that there was something terribly wrong with the whole story that Ruby shot Oswald, but the disease of Americana is very crippling.
7. And yes, the one masquerading as Ruby during the televised spectacle was FBI Agent James Bookhout. This is a comparison of the Garage Shooter with Bookhout when he was 26. They are the same man separated by 26 years of aging.
Notice that very fully cheeks, the very short neck, the distance between the eyes, the depth of the eyes (although they blackened out his eyes on the right to obscure him) but that is the same man separated by 26 years of time.
There is a great deal more that I could say, but I am going to cut this off here because it is enough to dwell on at one time. That Jack Ruby did not shoot Oswald is an absolute fact, and until a person realizes it, they are completely in the dark about the JFK assassination. There is no understanding what happened to JFK without understanding what happened to Oswald. I do not believe that Oswald was shot during the spectacle. I believe that was all theater. But, Oswald was shot afterwards, and he did die at Parkland Hospital that day.
