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. 

Poe wrote that his main impetus in work of fiction or poetry was to create a mood or feeling. As he put it in his "The Philosophy of Composition,"

I prefer commencing with the consideration of an effect.

In "The Fall of the House of Usher," the effect Poe wants to convey is that of an isolated, dark, sick, and claustrophobic environment and the impact that has on the people living in it. Imagery—description using the five senses of sight, sound, touch, taste, and smell—is vital to conveying this effect.

From the start, the description is relentlessly melancholy, with nothing introduced to lighten the mood. The narrator describes his approach to the house of Usher:

with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit. I say insufferable; for the feeling was unrelieved by any of that half-pleasurable, because poetic, sentiment with which the mind usually receives even the sternest natural images of the desolate or terrible.

In other words, the narrator is indicating from the start that there is no sense of the sublime—the mixture of awe and terror that suggests the grandeur of god—in the scene as he approaches his friend Roderick's home. Everything about it is uniformly and overwhelmingly dismal and bleak.

The dark, somber, joyless imagery conveys extreme nervous tension as the narrator enters the house itself and meets his old friend Roderick, now pale, sickly, and displaying hyper-sensitive frayed nerves and strange behaviors. The house has a "black oaken floor" and

Feeble gleams of encrimsoned light made their way through the trellised panes, and served to render sufficiently distinct the more prominent objects around; the eye, however, struggled in vain to reach the remoter angles of the chamber, or the recesses of the vaulted and fretted ceiling. Dark draperies hung upon the walls....

The darkness is unrelieved except by a lurid red light. Roderick himself plays "wild improvisations" on his guitar and paints strange, "ghastly," and "phantasmagoric " pictures.

All of this imagery conveys the foreboding gloom and perverse strangeness of the narrator's surroundings, setting us up for Madeline's bloody clawing from her tomb.