So it was admitted that the original photo showed a curved horizon.
But even if it had not been admitted, or even if the original had shown a straight horizon, there is a question - why does the rest of the earth, if flat, disappear beyond the horizon? Shouldn't we see all the of surface of the Flat Earth from such an altitude and angle? The fact that there is a horizon at all, and that the rest of the planet drops behind it, is evidence that the earth is round.