Edgar Allen Poe's use of imagery is essential in establishing tone in order to create a vivid world of horror both in mood and in plot. At the beginning of the story, the opening description of the dank tarn, and the fact that the narrotor feels, "an iciness, a sinking, a sickening of the heart" provides a dark and dreary world in terms of setting. To understand the Usher family, Poe describes a traditional family tree which is typically intricate; however, the Usher family tree "lay in the direct line of descent, and had always, with very trifling and very temporary variation, so lain." This imagery provides a description of the degeneracy of the situation: the Usher family has a history of inbreeding.
While the setting appears to be a dying world, so do the characters of the play: Roderick Usher is described as having a "cadaverousness of complexion; an eye large, liquid and luminous beyond comparision; lips somewhat thin and very pallid.." Poe provides an excellent description which captures many of the senses of imagery: sight, sound, taste, smell and touch. This Mr. Usher is not easy on the eyes because of his look of death.
By including a poem of rich imagery, "The Haunted Palace," Poe drives the reader to see the upcoming downfall of the Usher family. Where once the kingdom was described as having "banners yellow, glorious, golden" quickly becomes a place where "evil things, in robes of sorrow, assail[ing] the monarch's high estate;" This poem is an allegory, a story in which characters/setting represent abstract ideas or qualities. The poem's effect is one of foreshadow because it predicts the downfall of the Usher family.
Lastly, the demise of the Usher family comes to total fruition at the end of the story by the whirlwind. The narrator describes the scene as being, "long tumultous shouts sounding like the voice of a thousand waters...and the deep and dark tarn at my feet clos[ing] sullenly and silently over the fragments of the "House of Usher."
By the use of vivid imagery, Poe creates a story of terror and madness. Yet, the imagery he uses to describe the brother and sister surrounding the approaching climax does indeed help drive the plot as it is the psychology revealed in the imagery that develops and realizes the climax then moves the falling action. So, yes, imagery does indeed help move the plot.