Your starting point here is the definition of an "image." The term came into literary criticism with the "Imagist" movement, spearheaded by the early twentieth-century American poet Ezra Pound, who defined an image in his seminal essay "A Few Don'ts by an Imagiste" as "that which presents an intellectual and emotional complex in an instant of time." In other words, an image is a concrete description of something which has some sort of intellectual or emotional significance. It's not a symbol, which is intended to stand for something else (an icon for a computer application, a cross for the suffering of Christ, a flag for a nation), but rather something concrete which evokes both an idea and an emotion.
So, for example, the description of guns that "broke the chancel window-squares" is a simple concrete image of the shaking of the ground from artillery shattering the windows of a church. It has an immediate emotional impact because we think of churches as places that are quiet and peaceful. Intellectually, it suggests that warfare disturbs not only those at war, but also the old certainties of religion itself.
There are several images in this poem, often hyperbolic. We often say as a metaphor that something is loud enough to "wake the dead". Many of the concrete images of the work emphasize that World War I is so dire as to disturb not just the living but the dead; worms and mice are frightened, and skeletons sit up. The final image is that of the sound echoing not just across distance but across time to Camelot and even Stonehenge.