This kind of stuff sounds like, especially in the doses these workers are seeing it at, would be tough to stomach even for people who aren't hypersensitive. On the other hand, disturbing and violent images of the kind these people see all day everyday will result in emotional issues in probably a majority of people. A sociopath has a callousness of temperament that makes them, in addition to being able to stomach the most extreme imagery, also able to engage in the more directly harmful activity that leads us to call them sociopaths in the first: namely violent crime, robbery, etc. The sociopath, in addition to being "tough," is emotionally lacking where it's needed: the social emotions (empathy, guilt, remorse, shame, compassion, etc.) and the ability to form bonds with others, including family and sexual partners.