Two contrasting images might be the broken sculpture and the sand. One is man-made and in ruins; the second is natural and enduring. The irony of the poem is primarily found in the last lines, with the saying on the pedestal reading, "Look on my works, ye Mighty, and despair!" And the following line, which reads "nothing beside remains."
The poem is based on the Egyptian pharaoh Ramses II who did commission the building of many great works of architecture. However, in the war between man and nature, man and his works are ephemeral; nature continues on. The irony of the poem is that nothing of the pharaoh's works remain. And the pharaoh's words are only empty boasts. Even the sculptor's works are in ruins. The statue itself consists only of a "a shattered visage," and "trunkless legs of stone." Ozymandius, his works, his image all are conquered by "the lone and level sands."
The previous post was very thorough. I might also add that the speaker and the traveler from an antique land might also be images present in the poem. These speak to the fact that Ozymandias, as a king, has passed into legend. While he might have sought to be famous, he has actually acquired a sense of infamy in that he has become a cautionary tale of which people speak as more of a lesson than anything else. Consider the irony in that Ozymandias sought personal glory, to be dominant, to be "the king of kings." All that is left of him is a statue that is decrepit and broken down. In this light, Ozymandias becomes a warning to all other political rulers that those who rule for only personal glory will become nothing more than a footnote to history. The speaker and the traveler are images of this, for we only know of Ozymandias through word of mouth, through legend, through passing tale. In this light, these images help to feed the overall ironic tone that the poem strikes.