To get past the awkwardness of the uniqueness thing, I retroactively declare that we are looking for the unambiguously unique, physical, original object, which indeed in part represents a human, was created in a known year, etc. Clue: not many people have seen the original, and I can find no information about whether it even still exists.
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?