Diction is the writer's word choice style. Imagery is writing in such a way that the reader forms a mental image of multiple senses not just visual.
In "Contents of the Dead Man's Pocket," Jack Finney effectively creates suspense using both of these literary devices.
First, the imagery used, the mental image created for the reader serves to put the reader out on the window ledge with the main character, Tom Benecke. We can feel the "slight, chill breeze" and the rough brick surface. We can hear "his shoe soles shuffling and scrapping along the rough stone" and "the buttons of his jacket scraping steadily along the rough bricks." We can visualize the traffic moving along the street eleven stories below us. All of these things create suspense by putting the reader in the moment, creating a physical anxiety akin to what Tom feels.
Next, the author's diction, his word choice style, creates suspense. Written in concrete middle diction of daily conversation between educated persons, the sentences are long and flowing, stringing together several ideas and thoughts near the end to mirror the thoughts that a panic-stricken person might have. This creates a sense of urgency within the reader. Related to diction is vocabulary, and Jack Finney uses a long list of concrete, precise and vivid verbs (scraped, bounced, swayed, plunged, clung) that serve the above purpose, putting the reader in the moment to create anxiety and suspense.