Seeing one image once will have an impact but probably allow you to still lead a normal life after that. Spending all day looking at that sort of thing over an extended period of time is much more likely to have serious long-lasting effects.

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.