Our age is visually as well as if not more than, verbally oriented. Verbal images live in the consciousness and memory of the hearer. That is why preaching the parables and preaching as story have been two major themes in preaching in the last quarter-century. People warm to images. This has always been true, but it is especially true now.
Images allow for mind play.
Images do not control their own interpretation, but rather serve as the occasion for free association and interpretation in the mind of the hearer. Biblical imagery, especially that of the parables, has as much power as ever to enable people to form an understanding of God. At the same time, we must be clear that images are not the reality. They lead to the reality, but they are not one with it. Hence, a dose of contradiction may help preaching, e.g., you may want to argue that God is both the good shepherd and a fortress in the same sermon, so that people do not become wedded to one image.