what does"The effect was that of a Bertillon police poster, "mean?
Earlier in the morning five thousand cheap handbills had appeared on the streets of Dallas. At the top were two offset photographs of the President, one full face and one in profile. The effect was that of a Bertillon police poster, and it was deliberate, because the headline read:

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A police poster often shows people either arrested or wanted for arrest.  There are usually two photos or drawings - one face forward, the other from the side.  Showing the president front and side had gave the same effect as saying that this person (the president) is a criminal.