What is the most important imagery in "The Cask of Amontillado?"
The most important imagery in "The Cask of Amontillado" is the underground catacomb wine-cellar. With its dark, damp tunnels -- which seem both enormous and confining -- the cellar shows the strangeness of underground environments and Poe's underlying fear of being buried alive.
We had passed through long walls of piled skeletons, with casks and puncheons intermingling, into the inmost recesses of the catacombs.
(Poe, "The Cask of Amontillado," eNotes eText)
The wine-cellar is also a crypt, with generations of Montresor's ancestors interred within. Pointing out the skeletons and the nitre which forms on the walls establishes mood and increases the sense of claustrophobia; although the skeletons are long-dead, it is easy to be fearful. The catacomb is a place of death, not life, and the living should not disturb its silence.
This is an interesting and subtle question. You might ask, important for what? One very important piece of imagery is found in the story's fourth paragraph, when Montresor encounters Fortunato on the street: "The man wore motley. He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells." This is important because it is a very intense form of foreshadowing. The costume tells you Fortunato is playing the fool. You can expect Montresor to make him look stupid, as he does.
The next important piece of imagery is found when Montresor describes himself: "Thus speaking, Fortunato possessed himself of my arm; and putting on a mask of black silk and drawing a roquelaire closely about my person, I suffered him to hurry me to my palazzo." This is a man in a black mask and cloak. He blends into the night, and is, essentially, death. This further foreshadows the outcome of the story.
The final aspect of imagery that is important is the extended description of the crypts. From the spoken references to "nitre" to the descriptions of the damp and the bones, all of this is a pure horror setting, and a setting that can only end in death.