A new unlockable has been added to the gallery extras! A really cool artwork comissioned by a fan of the game, Coffy, drawn by Kamonabe Pasta. It has been added to the game with their permission. Big thank you to both!
You can unlock it by reaching the game's true end, it's the last one in the list (which unfortunately it's no longer a perfect square, but the sacrifice is worth it). If you already have, it will be unlocked automatically when you open the game.