A black flag with a skull and crossbones, but with an added white line running diagonally from the upper hoist to the lower fly, giving it a slightly tilted look.
This flag is a variation on the traditional Jolly Roger, and it reflects Jack's playful and somewhat chaotic nature.
It's worth noting that the "official" Jolly Roger used by pirates in the Golden Age of Piracy was a simpler black flag with a skull and crossbones, with no added details.
Do you want to know more about the history of pirate flags, or about other flags Captain Jack Sparrow has used?