Original: The cover is a metaphor for Bella's progression throughout the entire series; she began as the physically weakest player on the board, the pawn, but at the end she becomes the strongest, the queen.
Simplified: The cover of Breaking Dawn is a metaphor for Bella's changes during the entire story. She began as the weakest (at least physically, when compared to vampires and werewolves) player on the board: the pawn.