Imagery is the sense impressions an author includes, including what can be see, heard, felt, tasted, and smelled.
Some of the imagery in the story is often used to help Oskar remember about his dad. Such as when he can still hear his dad's voice, even though he wasn't present. He could also smell his dad's shaving cream in the house. He uses his senses to help remember his dad. This can also be a central message.