The image represents a major part of the creation of modern India, nearly the largest country in the world. The man in the image is the most recognisable and arguably most important in modern Indian history, and is known around the world. His influence and historical significance is enormous, which adds to the image's iconic-ness. More info on the Historical background of the image can be found here. The image has become one of the most famous photos of Gandhi, and is commonly used whenever he is brought up. His significant prevalence in culture as a major historical figure and symbol of peace movements segments this image as extremely iconic.