Pictures force us to collect our thoughts. They make us think about motivation, intent - they make us think about how we interpret our experiences, how we think about the world, how we try to understand the motives of others. (Maybe it's in our DNA. We look at pictures of other people and we want to know: what were they thinking?) And when it's a photograph of a crime or of violence, we think even harder. Such images make us care because they make us part of the mystery of what happened. We are not merely spectators; we are investigators. We are involved. What do the images mean? What do they show? What led up to these events? Are there mitigating circumstances? Is it as bad as it looks?
I wrote before about reading faces in pictures, and I think Morris is making the same point about how pictures make you work to understand what's going on. And Morris's points about denying reality are good, too; as I was watching this video on television, I couldn't believe that a camera crew was actually willing to follow the soldiers into such a dangerous situation. Anyway, good stuff.