John Keats was an English Romantic poet. Keats highlighted the use of imagery in his poetry to speak to the follow characteristics typical to the Romantic period. He valued feeling over reasoning (which was a characteristic of the Age of Reason because many of the characteristics of the Romantic period were developed their concern to move away from realism). Keats also typically focused upon the importance of nature and of the imagination.
As for the poem "Ode on a Grecian Urn", the imagery here is based upon the urn as the main "character" in the text. Here personification is loosely used to show the urn as a wisdom-giver to the narrator of the poem.
The urn is described by the narrator as being representative of a couple different things: a virginal bride and a foster child. The imagery on the urn represents more than a work of art for the narrator; it represents a teller of tales, a wisdom giver.
The depictions on the urn are similar to the descriptions of the urn itself. In the first stanza, the narrator calls the urn an "un'ravished bride". This means that the urn, feminine, has wedded, but has not completed marriage given consummation has yet to take place. Similar to this, the images of men and women on one of the sides shows a snap-shot of what is to come without depicting the scene has reached "consummation". The "Bold Lover", the trees, and the "fair youth" are stuck in a place where time has no control. All must stay locked in an act of "almosts" and "forevers".
In stanza three, a tree is depicted more descriptively. Again, like the lovers and the urn itself, the tree is frozen in a state of beauty- one where it will never lose its leaves or see spring to renew itself. Like the preceding stanzas, this side of the urn depicts a time and place where one is beautiful and will never see time change for it.
Stanza four seems to be the one in which the questions of the urn are asked. The narrator seems to be concerned as to why a sacrifice is depicted on the urn given the other imagery seems to speak to moments of happiness. Here, many questions are asked of the urn (who cannot reply and allow the narrator comfort with "her" answers").
The fifth, and final, stanza is where the narrator discusses the urn as a whole. Here, the narrator tells the urn that it holds secrets that will never be explained given the urn has no voice. Instead, the urn can meerly show pieces of a history in which the narrator finds only simply comforts given the many questions he is left with.
The last lines of the poem are by far the most famous of the text:
Beauty is truth, truth beauty- that is all
Ye know on earth, and all ye need to know.
These lines speak to the truth of the urn: beauty shows the truth and in truth one can find beauty. While the depiction of the sacrifice of the cow seems to be less than beautiful, the truth of the need for the sacrifice is where true beauty lies. The narrator does not need to know why the cow is being sacrificed. The narrator only needs to find beauty in the truth that the cow needs to be sacrificed.
Therefore, the imagery in the poem "Ode on a Grecian Urn" allows the reader to get a visual image of the urn. Without the imagery, the reader would not be able to understand the narrator's fascination with the piece of art. For the narrator, the urn represents more than a vessel to carry things in. It represents beauty in the world; and with this beauty comes truth--something the narrator seems to need in his life.
I would add the idea that the poem reflects an ideal form of love that is no longer recognized as such. Keats is depicting "courtly love," which was popular in the middle ages. Courtly love is an open display of affection that didn't ever need to be consummated. It was about lengthening that period of giddy infatuation as long as possible. It was this, head over heels, immature love that was seen as the absolute height of romantic being. So, the urn captures that moment right before the kiss, or, the initial physical consummation of mutual attraction. In other words, its all about the chase.
The importance of the static nature of the images on the urn cannot be overstated. Keats was obsessed with the transient nature of life and his own mortality. The urn represents something that can express beauty, or in this case the aestheic of courtly love, indefinitely. The urn, and the poem itself, reach back to the same idea in Shakespeare's 'Sonnet 18.'
"So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee."
It is also interesting to note that Keats was right to worry about death. He died, tragically, very young. He was like the Kurt Cobain of his generation.