Symbolic images stand for more than simply what appears before the eyes. They represent a deeper meaning behind the image -- something, for example, that an artist wants viewers to get from the image without saying it. In literature, symbolism is used to represent a deeper layer to the plot of a story. While the story tells literally what is happening, symbolism brings deeper meaning to the plot, setting and characters when used properly. Symbolic images do the same, portraying one literal moment in time, but possessing deeper layers of meaning behind them.
Consider the Christian cross of the Jewish Star of David. These are agreed upon images to represent a religious group. While the objects themselves are a cross and a star, they stand for much more for the religious groups they represent. Also, consider the image of the Virgin Mary. She is a symbol of purity to many.
Consider the Republican elephant and the Democratic donkey. These images are symbolic to the political parties they stand for. The donkey came from Andrew Jackson in his 1828 presidential campaign to make light of his critic's nickname for him, a jackass. He claimed the image of the strong donkey. Then, in an 1874 Harper's Weekly magazine comic strip, the donkey was shown warding off zoo animals, one of which was the elephant, which was labeled "The Republican Vote." These animals stand for the political parties. Thus they are symbolic images.
Visit an art museum to view many examples of symbolic images. If you have a curator, she will answer questions about the artistic symbolism, or simply read the literature next to the paintings. Artists have been inspired to use art as social outcry as François Millet did with his painting called "The Gleaners," a painting showing three black woman bent over collecting leftovers from a harvest. In 1857 France, this picture symbolized the rural poor and their marginalization, among other things, although it simply showed three women collecting a harvest.
Consider the deeper meaning in images when viewing them. Symbolism is a tool to make people think on a deeper, more critical level. View images for what they literally are, but then dig deeper and consider what else they could symbolize.