Ok, let's be more specific.
What would be the pictures (almost) everyone knows visually at the point they are now disconnected from their very origin and are used an emblems to illustrate various causes? Being a popular image is not enough, it has to have a broader meaning than what it was supposed to show originally (i.e., the image has now its own life).
Dorothea Lange picture is a good example as it represents the consequences of the economic crisis not from the Wall Street stanpoint but from real life. As such, people are very responsive to this image as they identify themself to this woman and I am pretty sure this picture will still be known in a hundred years.