The imagery in "Half-hanged Mary" invites readers into Mary's macabre and glorious imagination.
The first passage of imagery in the poem is gentle enough: Mary simply describes herself in her former life, when, with her "blue eyes," "sunburned skin," and "tattered skirts," she mostly minded her own business, curing "warts" and tending her "weedy farm." The images she chooses here make her sound hardy, skillful, and pragmatic: she may not have had much money, but she was eking out a living.
Sadly for her, she also has something that seals many women's terrible fates: a "sweet pear hidden in [her] body" (that is, female genitals). The "sweet[ness]" of this "pear" suggests that, as a single woman living on her own, she's both tantalizing and frustrating to the town's sexist men. But there's something poignant about this image, too: in a less violently misogynistic world, perhaps Mary would have been able to enjoy this "sweet pear" more herself!
Later, though, the poem transforms Mary into a different kind of fruit: a "blackened apple stuck back onto the tree." That "blacken[ing]" is both emotionally and physically evocative, suggesting that the tormented Mary feels she's being treated like a piece of rotting garbage—and that her face is literally dark from suffocation. Only the impassive moon stays to witness this suffering: it's "bone-faced," an image that makes it seem coldly white and deathly, and stands in contrast to Mary's own "blacken[ing]."
The women who come to stare at Mary, identical in their "dark skirts," are similarly unsympathetic: their "lipless" mouths and "upturned" faces only reflect their own fear. These flashes of imagery paint a portrait of huddled women in long dark dresses—an indistinguishable crowd of women too scared to help their former friend.
The tenor of the imagery changes when the sun rises the next morning and Mary gets cut down from the tree. Much of the imagery in the hanging section of the poem evokes darkness: there's a lot of stark black and white in there. Now, color and feeling creep in, and even synesthesia: when Mary sees the sun rise "huge and blaring," it's as if its light is an intolerably loud sound.
In her new, second life, Mary experiences the world in full color: she hears the "red-hot stars" speaking to her, looks at the world through "sky-blue eyes," and relishes mouthfuls of "juicy adjectives / and purple berries." All this bright, sensuous imagery suggests that her ordeal has given her a richer, stranger life than she had before: once she lived on a "weedy" farm in "tattered" skirts, but now even her dirty hands "gleam" with "holiness."