I've seen the same image. People have been theorizing that Crota (from the moon) is actually a dragon, specifically the one seen in the trailer, and that he's chained down under the moon. But a weird thing happened last night: while I was walking around Venus, I randomly looked up and saw the exact same silhouette, except now it wasn't black it was more of a reddish color. I really hope they're dragons, but maybe they're just Alien bird-type creatures.