Is it the photograph of the Tiananmen Square protestor and tanks?
Hail Radox! I note that the aforementioned Wikipedia has a "List of photographs considered the most important", though I'd think we're a few weeks away from recourse to that.
What springs to mind is either the photo of the Montparnasse train crash, although it doesn't have anybody on it, and the Conan Doyle 'fairies at the bottom of the garden' photo, but that has a whole person, not part of one, so I'm going to ask if the photo was taken before 1930?