The term picturesque in regards to literature translates to a piece of writing or speech that is strikingly graphic or vivid. The author's intention is to create a story with the use of detailed mental images.
John Keat's applies this to his poem, "Ode on a Grecian Urn." The poem tells the story of an unchanging world through elaborate imagery. Keat's describes nature and young love as unchanging, in the same way that it has been etched onto the urn. The young lovers will always share the anticipation of their first kiss, winter will never come, and overall the world seems to be an idealistic place.