Spurgeon, Caroline F. E. "Shakespeare's Imagery in Macbeth."
Shakespeare's Imagery and What It Tells Us. New York: Cambridge UP, 1935. 324-35. Rptd. in Twentieth Century Interpretations of Macbeth. Ed. Terence Hawkes. Englewood Cliffs, N. J.: Prentice-Hall, 1977. 13-21.
Thesis: Caroline Spurgeon was a pioneer in the study of Shakespeare's imagery, and she is still well worth reading. In this piece she discusses the following four "main ideas" expressed by imagery: She also mentions the idea of "the unnaturalness of Macbeth's crime" (19), "the action of rapid riding" (20), "the constant and recurring images of blood" (20), and "the images of animals, nearly all predatory, unpleasant, or fierce" (20).

Bottom Line: Good stuff.