Certainly the ethics of human cloning should be represented. There is currently not a global ban on human cloning, and less than a fourth of all nations have policies in place which make this practice illegal. What ramifications could human cloning have?
Never Let Me Go takes a look at questions that arise from the issue, including a debate on whether clones would have souls. (If God creates humanity and gives individuals a soul, then some might argue that beings not originating from God therefore do not have souls.) Should there be a global ban on cloning? If not, what limits should be in place? Artwork representing doubles or clones could convey this theme well.
Another theme is the examination of what it means to be human. Does humanity depend on a natural conception? Does it depend on looking like a human? Does it depend on a shared human experience? If so, what are those experiences? Kathy and her friends are kept separate and apart (this could be a theme in itself) and learn that many people do not even value their existence. To these people, their lives are meaningless and not worth getting involved in. Truly, they are seen as disposable. Does our society view as disposable any particular group of people? You could also represent this in your artwork.
A third theme could examine the issue of powerlessness which Kathy, Ruth, and Tommy face. Though they long to live, they find themselves fairly powerless to change the rules of the society they exist within. Both Ruth and Tommy are forced to undergo the organ harvesting which kills them. Images depicting oppression could reflect this theme well.
I hope this helps you begin to consider your ideas. Your teacher will likely be looking for a variety of images which convey a full knowledge of the book. Additional images representing a dystopian world, friendships, love, or art itself (as it is valued in their school) could also prove helpful. Good luck!