As I read this book, I asked myself the same question. The pictures seemed to sneak up on me, and were a bit unexpected and shocking. My answer or explanation for them is this: it seems to me that the author is using them to reiterate what is going on in the book at the time the pictures/art are inserted. Oskar is a bit disheveled, and therefore, the presence of the pictures don't make sense at first, either. Upon closer examination, you see the world through Oskar's eyes...a young man who is confused, wandering, trying to make sense of a world which was one day safe and full of the loving family to which he was so accustomed. Suddenly, that world changed into a dangerous place that can whisk away loved ones in a heartbeat without explanation or warning.
In a way, too, these pictures underscore the fantasy world that Oskar is viewing...a sort of magical realism...this is especially true for the falling man sequence of pictures at the end of the book. The man does not fall downward as one would expect...the pages, when flipped, show him falling upward. Toward the safety of the building which hasn't yet collapsed, or toward Heaven? It is in the eyes of the young boy...we can only make suggestions as to what Oskar is trying to do, explain, or carve out for himself and his mother in this new, more ominous existence. Oskar keeps imagining things in reverse, back to the time his father was still alive and leaving messages on the voice mail machine. I think that is what these pictures and art are doing for us...showing us Oskar's inner self and wishes...showing us the world as he wishes it were.