The Cartier Bresson snapshot of the soldier that just got hit by a bullet. That vietnamese girl, burnt by napalm running naked against the photographer. And of course that picture when the vietnamese police officer executes a man, when the bullet just have started to spread his brains out. Horrible, but these are also pictures that made some change through sheer impact. There are also a number of pictures from the nazi concentration camps.
It is so sad that i have to mention a more positive picture that was just as political and moved people around the world, the chinese man standing in front of the tank at tiananmen square. Talk about curage!