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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.
What is especially striking about Shakespeare's Sonnet 73 is that each quatrain contains a metaphor within a metaphor. For example, in the first quatrain he compares his time of life to early winter when the trees have lost almost all their leaves, and then he compares the bare branches of these trees with the places in churches where choir members sing, with the birds being implicitly compared to singers in a church choir. And in this same quatrain he uses a poetic conceit that the bare boughs are shaking because of the cold when in fact they only appear to be shaking from the cold but cannot really feel cold and can only be shaking because of the wind. It is a dazzling display of imagery. Mainly the quatrain compares late middle-age to winter and compares the bare branches of the trees in winter to choir lofts in churches.
In some of Shakespeare's sonnets he uses only one simile or metaphor, which usually makes the image stand out vividly. For instance, in Sonnet 29, which begins with "When in disgrace with fortune and men's eyes," he brightens the dominant tone of depression with the wonderful image:
And then my state
(Like to the lark at break of day arising
From sullen earth) sings hymns at heaven's gate...
The alliteration of "S" sounds in the last quoted line ("sullen," "sings," "hymns," "heaven's") enhances the image of a bird soaring and singing.