Jones, Aaronson, and Rutherford are very important to Winston because he saw a glimpse of true reality in relation to them. 

Working in the Ministry of Truth, Winston more often than not has to rewrite things the Party has said in the past. For this work, he usually requests reference materials that come to him through a system of pneumatic tubes. One day, the photo of Jones, Aaronson, and Rutherford comes across his desk seemingly by mistake and he realizes it is true evidence of reality outside what the Party depicts as reality. He throws the picture away down a memory hole but seeing it for a split second had changed him.

Winston's realization that the mind is a mutable thing able to be manipulated by the Party shows him that control of reality and thoughts are the Party's most effective tools. Common sense is heresy to the Party so the Party makes it impossible to practice common sense. No one can be entirely sure of anything because the mind can degrade and be fooled. As well, there is never any hard empirical proof of anything because the Party is always changing things so they are in line with the Party's current stance. 

If anyone was able to produce hard proof of the Party doing one thing or another, it would inevitably lead to the collapse of the Party as Winston knows it. As Winston asserts in Chapter Seven:

Freedom is the freedom to say two plus two make four. If that is granted, all else follows.

Freedom relies on the ability of people to make sense of their world as it really is and anything less is to be enslaved and oppressed. 

Therefore, the picture of Jones, Aaronson, and Rutherford showed Winston how invariably chained to the Party all its members are. The picture is what really made Winston aware of true reality and how the Party warps it as best they can. 

What this means -- what Winston is concluding -- is that the Party needs people to believe whatever it says.  It needs them and commands them to disregard what they know and to believe only what it tells them.

The case of the three men you mention shows this.  It is clear from the picture he finds that the men did not commit the crimes that they confessed to.  But this shows Winston how much reality has changed.   He and others who have his job had changed reality so much that no one would ever take this picture seriously.

When Winston sees the picture, he realizes how the rewriting of history makes true memory and use of common sense very difficult in this society.  People have to just rely on what the Party says.