IMAGERY designates a special usage of words in which there is a change in their basic meaning. Patterns of IMAGERY, often without the conscious knowledge of author or reader, are sometimes taken to be keys to a deeper meaning of a work. Some critics tend to see the "image pattern" as indeed being the basic meaning of the work and a better key to its interpretation than the explicit statements of the author or the more obvious events of plot or action. Contrasting images of light and dark are among the most conspicuous of IMAGERY.
Students in literature classes often get tired of teachers harping on how IMAGERY is used in literature to express meaning and theme, but it is this very thing that gives a work the depth that sets it apart from a simple telling of a story. And it is done so subtly by a skilled writer that it takes a careful reading, usually more than once, to recognize how the author uses IMAGERY.
(This post has been inspired by and in some instances, directly quoted from A Handbook to Literature, 8th Edition, by William Harmon and C. Hugh Holman)