The primary image in "The Raven" is the picture of the raven sitting above the bust of Pallas above the speaker's "chamber door." Through this image and other diction choices associated with it, Poe creates an ominous mood in his poem.
Ravens are associated with death because they are scavengers, often feasting on carrion. A black bird, like a black cat, can be considered an omen of bad luck. In the Bible, birds often symbolize evil; for example, in Jesus' parable of the sower, the birds come and steal the seed of the gospel so it cannot produce salvation. In that case, birds represent demons or the devil. Having the unpleasant bird sit above a bust of Pallas, the goddess of wisdom, is even eerier because it gives the omen gravitas. If one sees the raven in the same frame as the symbol of wisdom, it implies that what the raven represents is true. The bird's position elevates its import; since it is above the speaker, it takes on more authority, and the speaker seems subjected to it.
Poe builds on this powerful image by associating it with words and phrases that add to the dark and disturbing tone. It comes from the "night's Plutonian shore," or the Underworld. The speaker imagines a doleful backstory for the creature: "unmerciful Disaster followed fast and followed faster." Other word choices that create a depressed and/or eerie mood include "ominous," "dirges," "melancholy burden," "desolate yet all undaunted, on this desert land enchanted," "demon's," and "devil."
Using a powerful image with an unmistakably ominous connotation and unforgettable diction, Poe crafts a story in verse that is hauntingly dark.