They say a picture is worth a thousand words. That being true, all the years of photography have provided an almost infinite number of those words. Many pictures are so powerful that they speak to generation after generation, making them easily recognizable by the general public. Images such as these are often referred to as iconic. But what causes an image to go from the camera to being a cultural icon? I think it is because some images are powerful enough to evoke certain emotions that many of us can relate to experiences or desires in our lives. The four main emotions evoked I believe are: those of certain eras, events, person/people, and artistic/shock value.
This photo of a soldier coming home after World War II is sweet, but also is an iconic figure of the beginning of the baby boom and the generation of children born during that time. It also signals the end of the WWII era, perhaps more importantly. But that is one cool thing about iconic images: they mean different things to different people. Baby boomers and generations after will probably see the photo representing my first idea, while WWII veterans will probably think of my second idea.
Some images are iconic simply because of the event they capture and the way they capture it. This image has been for many a source of pride, remembrance, controversy, and other emotions. The pride and remembrance are felt by people who were and are shocked and amazed at the accomplishment mankind made that day. The controversy is felt by people who wonder if the event actually happened or was staged to scare the Russians. Whatever emotion it evokes for a person, it is the larger event surrounding this photo that puts it into cultural history.
Some images are iconic because of the person the image is taken of. The above example of Blessed Mother Teresa of Calcutta is iconic because it evokes the memory of one of the most influential people of the 20th Century and an eventual saint in the Catholic Church. Other people are iconic for their image alone because of bad things; but they are nonetheless famous and easily recognizable by most people.
Lastly, some images are iconic because of their artistic and/or shock value. This image has been the source of much controversy. Is it a photo of a real sea monster or a clever hoax? Whatever the case, the idea of this monster has made Loch Ness a famous location and has spawned various forms of fiction dealing with sea monsters.
There is probably an anomaly in the vast sea of iconic images; but I think these four categories pretty well sum up why certain images become iconic.