I usually assign this project as well. My focus is to get students to think about the symbols in the book. What do you think is the most important symbol in the book? Then choose one for your cover. Some options you might try are: apple, bicycle, hair ribbon, needle, sled, grandparents, babies and pills. You can use any of those as a symbol of the oppression and control ever-present in the society.
I think you can go online and find a ONE WAY street sign and make that the cover. After all, that is the embodiment of the sense of community in The Giver, it is the way society wanted people to act, think, and be, and it might also bring in a sense of understanding the main idea.
I think what you use to represent the community should depend on what you want to say about the community. Are you trying to portray it as a good place -- is this like an advertising brochure?
If it is supposed to portray the community as a good place, I would suggest something like a picture of a couple of adults with two kids. That could make it look nice -- like it's a good place for families (even though there isn't much family there). Or you could have a picture of someone at their work looking happy. This would work because everyone has work and it's always work that they are assigned on the basis of what they're good at.
I believe that you could use bland gray color and just pencil tones to represent the lack of color in the society. Maybe you could draw some children with the same features and looks by dull buildings with streets and no trees or flowers. The children could all look alike except for one who could have blue eyes. His eyes could be he only thing of color on the whole page. He would represent Jonas. The community was the same for everyone but had people in the streets who did different jobs. There was no identification of color. Yet, we know that Jonas has eyes that are not dark like the others. We later learn that he and the baby Gabriel have blue eyes.