A lot of these symbols are of the really in-your-face variety (like the train—hey-o!). Then there are others that you wouldn't catch unless you were really looking, like the picture in Arthur's dream during the Saito job.
What picture, you ask? Our point exactly. It's the picture in the room Cobb takes Mal to before he tries to break into the safe. She's looking at a picture and comments that they must be in Arthur's dream… but Cobb denies this saying "actually the subject is partial to postwar British painters." Of course, Cobb is merely denying this to himself— he is talking with a projection of his own subconscious, after all.
First, let's get a little art background. The painting in question is Francis Bacon's Study for Head of George Dyer.
Why this painting in particular? Well, there's hole in the middle of a painting of George Dyer's head. Bear with us: Dyer was a friend and lover of Bacon's. He was also an alcoholic, and he came from a criminal past. Dyer ultimately died of an overdose but during the years he was Bacon's buddy he served as an inspiration for the artist.
What does this have to do with Mal and Cobb? In some ways, Mal is the Dyer to Cobb's Bacon. Mal is just a projection of Cobb's subconscious just like the painting is Bacon's representation of Dyer. Dyer's death greatly affected Bacon, and he continued to paint figurative and emotionally charged portraits of his late lover. In fact, his portraits of Dyer have become some of his most critically-acclaimed works.
Hmm. That sounds… familiar. Much like the painting of Dyer, the projection Mal isn't like the real Mal: she's really just a shell of her former self, hollowed of her personality. Her personality has been replaced by vindictiveness, which stems from Cobb's guilt and his idea that his wife would (and even should) desire revenge on him. Cobb to has continued to keep Mal alive by constantly recreating her in emotional and often violent dreams.