Who do they remind you of?
Just as the Three Stooges were choreographed, the three detectives were choreographed. They were squeezed into that picture.
But, why was the flipped version preferred?
Well, for one thing it turns Oswald's clenched fist into a clenched RIGHT fist. Wasn't his right hand Oswald's dominant hand? Aren't most people right-handed? So, it looks more defiant for him to be doing it with his right hand. Plus, it centers Oswald better in the image and makes him more the subject of the picture. Just as you read from left to right, you view from left to right. So, your eyes get to him before they get to the others. I can see why they would think it's more pleasing to the eye and makes Oswald the focus better.
Apparently, it is a common practice in political propaganda to look at and consider the flipped image, which may be preferred for numerous reasons.