Early in the letter, Dr. King explains his presence in Alabama and why he has come from Atlanta, Georgia, to Birmingham to assist in the fight against injustices there. He explains that he is not an outsider; in fact, he says, "We are caught in an inescapable network of mutuality, tied in a single garment of destiny." While this is a metaphor, comparing the relationship between Dr. King and the clergy to whom he speaks to an item of clothing, or a piece of cloth into which they are all interwoven, this also creates a visual image of the "garment."
Dr. King continues, explaining the need for "nonviolent gadflies," actions which would, like gadflies, pester and buzz, prompting social change via good-faith negotiation (another metaphor). The use of this phrase conjures a visual image of the flies themselves, and perhaps even an auditoryimage too—their buzzing is "nonviolent" but irritating enough to provoke some response. Further, he hopes that the tension created by the catalyst of nonviolent protest "will help men rise from the dark depths of prejudice and racism to the majestic heights of understanding and brotherhood." These visual images might compel someone who implicitly condones prejudice by failing to work against it to actually begin the hard work of climbing from the "shadow" up into the light. We might imagine the visual image of a mountain: prejudice lies in the shadow, and "understanding and brotherhood" await at the peak.
Dr. King also says that "The nations of Asia and Africa are moving with jetlike speed toward gaining political independence, but we still creep at horse and buggy pace toward gaining a cup of coffee at a lunch counter."...
Thevisual images of a speeding jet versus a much slower horse and buggy are easy to picture. He draws attention to how slow racial progress is in America in a way that makes the country seem technologically behind the times, a comparison that white Americans might more easily understand.