The image of the man falling from the World Trade Center is probably the most troubling one that Oskar (and all of us) saw from the countless gruesome images of the 9/11 attacks. It brings this massive tragedy down to a horribly intimate scale. Oskar's terrified of the image but can't stop thinking about it. At the same time it gives him a strange comfort, because he thinks it could be his father and he believes that falling to your death is less painful than burning to death.
Oskar's tried looking at this photo from very close, but concludes that the closer you look, the less you can really see, like how images can look pixilated when seen at very close range. This thought reminds the reader of something his mother said about his father—that sometimes Dad couldn't see the forest for the trees. Is the author telling us that Oskar's obsessive focus on the falling man keeps him from seeing the bigger emotional picture of his loss?
The falling man is the ultimate image of hopelessness. We know with 100% certainty when we see this image that the man will die. The only solution is illusion. Oskar makes a flipbook of the falling man and reverses the image in an attempt to replace terror with comfort.