Imagery, as the name implies, is a description in accordance with human senses, often with emphasis upon certain details or interpretations which associate the imagery with emotion. The same scene, depending upon the author's attentions, can be described differently according to the mood the author wishes to set.
Many of Irving's stories use imagery to tell us about his characters;
A miserable horse, whose ribs were as articulate as the bars of a gridiron, stalked about a field where a thin carpet of moss, scarcely covering the ragged beds of pudding stone, tantalized and balked his hunger; and sometimes he would lean his head over the fence, look piteously at the passer by, and seem to petition deliverance from this land of famine.
Through this imagery, we may understand that Tom Walker is not the best of men, neither responsible nor successful. We might have simply been told this, but the details of this imagery, particularly (in my opinion) the horse's ribs, convey a sense of pity and sympathy for the animal; we are already disposed to find Tom Walker unlikable.
Like most short cuts, it was an ill chosen route. The swamp was thickly grown with great gloomy pines and hemlocks, some of them ninety feet high; which made it dark at noonday, and a retreat for all the owls of the neighbourhood. It was full of pits and quagmires, partly covered with weeds and mosses; where the green surface often betrayed the traveller into a gulf of black smothering mud; there were also dark and stagnant pools, the abodes of the tadpole, the bull-frog, and the water snake, and where trunks of pines and hemlocks lay half drowned, half rotting, looking like alligators, sleeping in the mire.
Irving tells us, and then shows us, that the path through the swamp was a poor choice. Rather than saying that it was difficult to walk through, disgusting, or dangerous, he articulates it as a problem made evident merely by observing it.
The imagery diminishes later in the story, and is replaced by dialogue. This is largely because the scenes and character personalities do not change significantly once introduced, and it is assumed that the reader recalls the mood as painted by the imagery; when Tom talks to the Devil, we know he is in a gloomy swamp, and we don't need to be reminded of it each time.