For one, it subtly draws attention to the hat, which draws your eyes away from the center of the image with all the weird stuff which they don't want you to examine too closely. Is that really Oswald's left arm? Did he really have such a humongous hand and thumb? I better compare it to other images of his hand.
They don't want you doing that. They want you to just take in the story of what the photo is telling, but without scrutinizing it.
Oswald did NOT slap his arm to his chest after being shot. And I am not suggesting that he didn't. I am telling you, in no uncertain terms, that he did NOT. Nobody would. Nobody ever has. Nobody ever will. It's just a stupid, childish, over-dramatization. And I'll give you an example. Here is Uma Thurman play-acting after her daughter Bibi play shoots her in Kill Bill Volume II.
You see. Oswald is doing the same thing. "You got me!"
But, childish, stupid people think it's for real.
It's just play-acting in both cases, except that with Oswald, it's something that the photo-alterers imposed on him. It's THEIR childishness, not his. It's their over-dramatization, not his. Sticking that arm in there was their idea. It was their doing.
And, the hat, with its festive grey vertical stripe, distracts from scrutinizing it. And that's why they dressed it up; to draw your eye to it.
Again: nobody who was shot in the abdomen through the rib cage would react by slapping his arm to his chest. Nobody. Nobody ever does. Nobody ever has. Nobody ever will. It's just stupid. Stupid people acting stupidly.