Imagery is the use of figurative or descriptive language to create a vivid mental picture. It involves at least one of the five senses--sight, sound, touch, feel, taste. Imagery in "The Masque of the Red Death" is ghastly.
Example of Imagery - "There were sharp pains, and sudden dizziness, and then profuse bleeding at the pores, with dissolution. The scarlet stains upon the body and especially upon the face of the victim, were the pest ban which shut him out from the sympathy of his fellow men." (145).
Analysis - Poe establishes the mood and setting of the story with the vivid description of the Red Death. The passage establishes the horror of the disease and explains why the guests would react to the blood stained intruder at the end of the story.
Imagery in the 7th Room
Example of Imagery - "The seventh apartment was closely shrouded in black velvet tapestries that hung all over the ceiling and down the walls, falling in heavy folds upon the carpet of the same material and hue" (146).

Analysis: The oppressiveness of the 7th room contrasts the gaiety of the previous six. The darkness of the room and the heaviness and darkness of the curtain symbolizes death. No wonder none of the guests wish to come near it. Note the pun on "shroud."
Example of Imagery - "The panes here were scarlet--a deep color....In the corridors that followed the suite, there stood, opposite to each window, a heavy tripod, bearing a brazier of fire, that projected its rays through the tinted glass and so glaringly illuminated the room. And thus were produced a multitude of gaudy and fantastic appearances...The effect of the firelight that streamed upon the dark hangings through the blood tinted panes was ghastly in the extreme" (146-7)
Analysis - So much for peaceful death. The images created by the brazier of fire and blood tinted glass give the room of death a ghastly appearance. The gaudy and fantastic