Imagery in literature is used to paint a mental image of something. The techniques used are descriptive and paint a picture that allows the reader to visualize the setting, person, or image that is intended to be conveyed.
An example of this would be: The old farm encrusted with barren soil and remnants of long decayed crops stood lonely and isolated as the wind pounded its walls.
Symbols are when w writer uses an object or idea to represent something else. It is usually something that the author feels is significant in the life of the person or central to the stories theme.
A good example of symbol use is found in the book "Lord of the Flies." The boys in the story take a pig's head and stick it on a stake that is pointed at both ends. Then the head is lodged into the ground. The head represents a religious icon as the boys move away from civilized behavior. It also represents the savage behaviors that the boys engage in.
Imagery is descriptive language that allows you visualize everything that is going on, where as symbol is usually an object or an action that stands for something and has a deeper meaning then what is simply written in words in the book.