How did Edgar Allan Poe use imagination in "The Fall of the House of Usher?"

Imagination is central to Poe's story. First, the reader is invited to engage in Roderick Usher's insistence that his home is a living and evil organism:

He was enchained by certain superstitious impressions in regard to the dwelling which he tenanted, and whence, for many years, he had never ventured forth -- in regard to an influence whose supposititious force was conveyed in terms too shadowy here to be re-stated -- an influence which some peculiarities in the mere form and substance of his family mansion, had, by dint of long sufferance, he said, obtained over his spirit -- an effect which the physique of the gray walls and turrets, and of the dim tarn into which they all looked down, had, at length, brought about upon the morale of his existence.

In other words, the house itself has developed an oppressive personality that is making Roderick sick. Roderick imagines that inanimate objects, like the house, are alive:

This opinion, in its general form, was that of the sentience of all vegetable things.

Second, Roderick himself is overly affected by his imagination, such this his nerves and senses are on edge from a "nervous" affection that: 

displayed itself in a host of unnatural sensations. Some of these, as he detailed them, interested and bewildered me; although, perhaps, the terms, and the general manner of the narration had their weight. He suffered much from a morbid acuteness of the senses; the most insipid food was alone endurable; he could wear only garments of certain texture; the odours of all flowers were oppressive; his eyes were tortured by even a faint light; and there were but peculiar sounds, and these from stringed instruments, which did not inspire him with horror.

Third, the reader is asked to "suspend disbelief" in accepting that Madeline clawed her way out of her tomb and her crypt. 

Finally, the story poses the question: is Madeline real or a projection or figment of Roderick's fevered imagination? The story critiques imagination run amok in wild and perverse ways, as represented by Roderick, yet pulls us in as readers with its powerfully imagined setting and consistent atmosphere of brooding oppression.