Imagery can be a difficult kind of figurative language to discuss because it is varied and encompasses a lot of other types of figurative language. Basically any kind of figurative language that helps create an image in the reader's mind is imagery. This image can be visual (what you see), auditory (what you hear), tactile (what you feel), gustatory (what you taste), olfactory (what you smell), kinesthetic (describing movement), or organic (creating a feeling or emotion).
The first striking and memorable use of imagery is that describing the Ancient Mariner himself, in the third line of the first stanza: "'By thy long grey beard and glittering eye," which tells the reader that the mariner is old (grey), unkempt (long... beard), and intense (glittering eye.) The mariner's "glittering eye" is mentioned again in the fourth stanza, and in the fifth stanza he is described as "The bright-eyed mariner." This repetition makes the mariner's eye his most striking feature. The word "glittering" shows that the mariner, although old, has a quick intelligence and a mesmerizing story to tell. His bright, glittering gaze seems almost hypnotic, which helps the reader to feel what the character of the wedding guest feels when the mariner stops him.
That brings us to the story. The mariner begins by describing how the ship crossed the horizon line, obscuring the view of civilization, and then how the sun rose and fell, going higher and higher each day, as the ship sailed southward. In this part of the poem, Coleridge used personification to describe the sun:
The Sun came up upon the left,
Out of the sea came he!
And he shone bright, and on the right
Went down into the sea.
Personification is a kind of imagery. It ascribes human qualities to an inanimate object. Coleridge makes the sun seem like a character by referring to it using the pronoun 'he.' This is the first instance of the strong imagery of nature in the poem, and it also foreshadows the supernatural elements that will be introduced later in the poem.
The storm is also personified:
And now the STORM-BLAST came, and he
Was tyrannous and strong:
He struck with his o'ertaking wings,
And chased us south along.
This makes the storm also seem to be part of a sentient force of nature that surrounds them, and makes the ship and the mariners seem small and powerless in comparison.
The next stanza has another kind of imagery, a simile: "As who pursued with yell and blow / Still treads the shadow of his foe, / And forward bends his head," which compares the ship to a man running from a huge predator, the storm.
The next several stanzas are filled with imagery describing the Antarctic sea filled with ice. For example, "The ice was here, the ice was there, / The ice was all around:" uses repetition to highlight the sheer amount of ice surrounding them, and "It cracked and growled, and roared and howled," uses aural imagery to show the sound the ice makes.
The most famous lines from "The Rime of the Ancient Mariner" are excellent examples of striking imagery:
Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.
The repetition of "day after day" highlights the long period of time during which the ship is stuck on the still sea and gives the reader a feeling of the passage of time; and the simile, "as idle as a painted ship upon a painted ocean" creates a visual image of the stillness of a ship in a painting.
Water, water, every where,
And all the boards did shrink;
Water, water, every where,
Nor any drop to drink.
More repetition creates an image of the small ship stranded in the wide, endless sea, while "and all the boards did shrink" means that the wooden boards constructing the ship dried out so much that they shrank, another visual image of the ship's idle state in the calm sea with absolutely no precipitation. When you read these lines, you can feel the thirst of the stranded mariners!
"The Rime of the Ancient Mariner" is absolutely filled with striking imagery like this. You can read the poem in its entirety on eNotes here and find out more information about it here.