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?
[P] Not Miss Napalm 1972. Remember, this was not created to be a record of something. [RtG] Not a record sleeve, or anything else to do with "records" in that sense.
Sorry, I missed Boolbar and Simons: [B] The person's face is visible. (Knowing murmurs from the audience.) [SM] Not scientific (but approving murmurs from the audience).
[SM] It is not, but that's on me for not having any previous knowledge of a landform called Lord Hereford's Knob. [b] It...certainly protrudes from it, but no, more localised than that.
[blamelewis] Aye! It is indeed Arthur’s Seat. Take these bagpipes and a sturdy pair of hiking boots, and either play to the wind or chuck them off the top, I guess.
[Tuj] Not aware of any of them going by Arthur. [Raak] Not an individual. [Projoy] 25 more guesses. [Simons] Possibly, but it's not a regular part of the human diet. Also lots of people would be quite cross.