Imagery is comprised of descriptions of information that appeals to our senses; as such, it can be visual (sight), olfactory (smell), tactile (touch), gustatory (taste), and auditory (hearing). The first instance of imagery that I find particularly arresting is created by O'Brien's description of his work at the Armour meatpacking plant in his hometown. He says,

After slaughter, the hogs were decapitated, split down the length of the belly, pried open, eviscerated, and strung up by the hind hocks on a high conveyor belt. Then gravity took over. By the time a carcass reached my spot on the line, the fluids had mostly drained out, everything except for thick clots of blood in the neck and upper chest cavity.

There's obviously quite a bit of visual imagery in this quotation, making it incredibly easy to imagine the scene. Phrases like "split down the length of the belly," "pried open," and "eviscerated, and strung up" make for particularly gruesome visuals. Even "thick clots" and "chest cavity" help to convey a gory, vivid scene. O'Brien is not a squeamish person, and he is not someone who might choose to avoid war for some reason of that nature.

A not dissimilar image is created with O'Brien's description of Elroy Berdahl. O'Brien says,

His eyes had the bluish gray color of a razor blade, the same polished shine, and as he peered up at me I felt a strange sharpness, almost painful, a cutting sensation, as if his gaze were somehow slicing me open.

This arresting image actually seems to combine the visual with the tactile. We can imagine the sight of the color and shine of the old man's eyes, but the "sharpness" and the "cutting sensation" make O'Brien feel as though the old man's gaze was "slicing [him] open," like the pigs. It seems to render the narrator helpless and vulnerable, just like the pigs he helped to process at the plant.