When we have enough years between us and a historical event, it becomes a littler easier to get the gist of what happened during it.
For instance, although both World War I and World War II likely seemed chaotic and unpredictable at the time, it's not difficult to get a general sense of how they began, progressed, and ended. Of course, we know we could drown in information about both of those times if we went in for a deeper study but the point is that we could do more than stare blankly if someone asked us about them.
However, that doesn't mean that a historical photo can't completely take us by surprise. And when we do find out the story behind it, it can give us a whole new perspective on what we thought we knew.