I would say Chinese took these photos for their internal records. However, they did not want executioners to be identified. Two guards wore sunglasses to do that, but the one in the center apparently did not. So they blacked out his face. I suspect that some underground organization managed to smuggle photos out. There are some secret organizations inside China. For example, Falun Gongs who were forced underground. Some of them could bribe police. I am not saying that this is how it happened. I am just saying that smuggling out such photos is possible.
That's a logical and reasonable answer to how we got it. I know these things some time happen. The person or persons doing the smuggling would be shot as well.
The other part of my question, the amount of damage,still puzzles me.
I know that one 7.62x39 cannot do the damage we see. I am very skeptical about this.