Concerning your question about "When I Have Fears" by Keats, identifying images and specific language in the poem is just the first step to analyzing images and language. What matters more is how these contribute to the work as a whole.
This Elizabethan sonnet is, of course, divided into three quatrains (four-line stanzas), followed by a closing couplet (pair of rhyming lines).
The first quatrain uses diction associated with growing crops to create the image of future books of poetry stacked in a storehouse: gleaned, teeming, garners, full ripened grain. The speaker's poetic ideas transformed into books are like the growing and harvesting of crops. And his worry about dying before he can finish all that he wants to write, is like a farmer worrying that his crops, his livelihood, will fail.
The second quatrain moves from the fecundity of nature to the romance of nature, and the speaker worries that he will die before his imagination fully develops and he is capable of tracing their shadows (figuratively speaking) with his poetry. The metaphor of tracing shadows is also an image, making concrete his act of writing.
The speaker's anxiety shifts from his future fame as poet to his future as a lover in the third quatrain. He worries that he will die before he has loved a woman, and been loved by her. He may never look upon a "fair creature," or "relish" in the "fairy power" of love.
The three stanzas begin: "When I have...," "When I behold..., "And when I feel...." The thoughts begun with these subordinate whens are completed by the main clause in the form of an image:
Of the wide world I stand alone, and think
Till love and fame to nothingness do sink.
This closing couplet reveals the speaker feeling alone, and his hopes for a remarkable life dimmed. If he doesn't have time, he has no hope of fame and love.