What aspect of the crime scene as observed by Al Dewey examining photographs foreshadows the eventual capture of the suspects?

When looking at the photographs, Dewey hoped the he "might see something." Dewey described it as one of those instances where you're trying to find the hidden animals. I might compare it to Where's Waldo. The aspect of the photos that foreshadows the capture of the killers would be the footprints he spots in a photo of Mr Clutter........ diamond-patterned soles; something not visible to the eye, but something visible on film because of the flashbulb used to take the photograph.