It feels like you are suggesting that under a certain amount looking at images of children being sexually abused or tortured shouldn't have an impact on mental health? For me personally I think even a single image like that would haunt me.
I don’t think there’s a magical line where it suddenly has an impact on mental health, and the details will vary from person to person, but there is most certainly a point where you go from functioning adult to justifying a formal diagnosis of PTSD or something similar.