link: u.nu/gxQRN.
There are many qualities an image must have in order to be considered "iconic".
The requirements for an iconic image fall into a few categories:
A recognizable image is something that can be easily identified by anyone within a culture or subculture. An image has to stick in people's minds and be relevant in order for it to be widely-known enough to become recognizable.
Cultural significance means influential or representative in a cultural context. An image that's culturally significant has to be important in relation to its associated culture. Often it's representative of a major event within a culture or for the culture as a whole.
An image is symbolic when the viewer derives more meaning than what can be bluntly seen on the image. Symbolism usually requires history or greater ideas to be related to the image, either directly or indirectly. A symbolic image will make the viewer think about things related to the image, while a non-symbolic image will make the simply think about the image's composition and nothing more. What can be considered symbolic is subjective.
A timeless image is something that is achieved when an image is recognizable for an extended period of time. It's recognizability exists independent of time, and will be viewed as significant no matter when it's viewed.
This image of Mahatma Gandhi is incredibly iconic. In it we can see Gandhi sitting peacefully in a room reading next to a spinning wheel. Those more familiar with the context of the image may know that the room is actually a prison cell. In the photo, Gandhi looks focused and peaceful. His presence seems to be the main part of the image, yet only takes up about a third of the frame. The rest of the space is taken up by the spinning wheel to his right, which is symbolic in its own right, and is a commonly associated symbol to Gandhi. A lot of meaning and history is connected to the image, adding to its iconic-ness.