Although come to think of it, yes they should have been the pirates that Gybrush recruited for the voyage to Monkey Island(TM).

Also, did you ask Mr. Purcell who they are? Surely he'd remember.

As a comment to unwesen - I think the image captures the game very well. The skull and crossbones represent pirates - well and also evil, and the fight against evil.. by pirates. It's an irony. The ghost ship, the cannibals, all have their place too. It doesn't have to capture the game exactly - just enough to draw potential loosers with no life (and parents with no money) into buying it. It's a pirate adventure and that's what's shown in the painting done by Purcell.