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In terms of imagery in Shakespeare's Sonnet 73, there are several examples.
First of all, imagery is a description with words that creates a vivid image in the reader's mind. Imagery can be presented in the form of similes, metaphors, allusions, etc. Often sensory details are used.
...imagery includes the "mental pictures" that readers experience with a passage of literature.
The second line of the sonnet shows imagery that makes one envision autumn, not just with the color of the leaves, but the dwindling number or absence of them:
When yellow leaves, or none, or few, do hang...
The next line appears to use personification, describing tree limbs that seem to "shiver" with the cold. Only people (and perhaps animals) shiver with the cold—not trees.
Upon those boughs which shake against the cold...
The following line is a metaphor, comparing the tree limbs to "choir" lofts where choirs (birds) used to sing:
Bare ruined choirs, where late the sweet birds sang...
This next line brings to mind the image of colors of the sky during a sunset:
As after sunset fadeth in the west
As the poem progresses, another metaphor is used as the life within the speaker is compared to a "glowing" fire:
In me thou see'st the glowing of such fire
These lines provide imagery—in reading the entire poem, and paying special attention to the images within, the reader can better understand the true message (the theme) within the poet's words.