I have no illusions about the completeness of the definition I just suggested. It's short and snappy and I think is at least one characteristic of art. And I don't mean that the metaphor must be particularly deep or come with some kind of agenda. It's enough that the artist moves outside the commonplace and gets you to pay attention by showing you something you haven't seen before.
I've chosen to post M. C. Escher's "Day and Night." Probably the first aspect of this print to jump out and grab a person is the contrast of black birds in white sky and white birds in black sky. On closer inspection, the it is nearly symmetrical except for the inverted color scheme. This print explores the duality of light and dark: how can you understand one thing except with reference to a complementary notion? You only see the black birds because of the contrast with the white sky (and vice versa).
At the same time, this image reminds us that such dichotomies need not be totally well-defined in the first place. Where does the day end and the night begin? What is foreground and what is background? That is, how many birds are in this picture?