Imagery refers to an author’s use of descriptive language to vividly depict settings, characters, objects, events, or ideas. Imagery need not be only visual; it can include any kind of sensory detail that appeals to one of the five senses. (These five types of imagery are referred to as visual, auditory, olfactory, gustatory, and tactile.) The key part of a passage that demonstrates imagery is the uncommonly descriptive language, which often also includes figurative language such as metaphor, simile, or personification.
“Maycomb was an old town, but it was a tired old town when I first knew it. In rainy weather the streets turned to red slop; grass grew on the sidewalks, the courthouse sagged in the square. Somehow, it was hotter then: a black dog suffered on a summer's day; bony mules hitched to Hoover carts flicked flies in the sweltering shade of the live oaks on the square. Men's stiff collars wilted by nine in the morning. Ladies bathed before noon, after their three-o'clock naps, and by nightfall were like soft teacakes with frostings of sweat and sweet talcum.” (To Kill a Mockingbird, Ch. 1)
It isn’t enough to identify passages that simply offer a basic description of the scene (ex: “The house sat atop the hill. The lawn was large and green”); visual imagery must create a vivid and detailed picture. Notice that in the above passage, Harper Lee doesn’t simply say the weather was hot; rather, she describes the heat through specific, vivid images that allow us to imagine the feeling of the heat and visualize its effects.