It's the reference example for what a "shock image" is. How in the blazes is the choice of that particular image over practically anything else based on anything but its capacity at causing distress?
The blog post both acknowledges it as such, and even says "Let this be a lesson to you", making it clear that its use is specifically meant to be go beyond stopping people and actively punish them. And even if it weren't for that straight up confession, it'd STILL be patently obvious that this is what they are doing. We are in clear-cut "shotgun booby trap" territory here. As others have pointed out, there are subtleties at play over who is ultimately responsible, but the author's intent to cause distress is not in question in any way shape or form.
As much as it's fun to see vigilante retribution like this (and boy is it!), that does not discount the obvious legal exposure at play.