also, i did think this was a bit US-centric but I think a lot of people in the comments are suggesting not 'photographs that changed the world' but 'photographs of events that changed the world'.
iwo jima, 'tank man' and the burning monk should have been included though. I also suggest these two:
http://en.wikipedia.org/wiki/Earthrise
and perhaps to a lesser extent
http://en.wikipedia.org/wiki/Pale_Blue_Dot

hahahahaha Matt, good call. should definitely be on there. goatse is the very embodiment the modern internet age!