It's been a while but a new cheat has been discovered! Thanks as usual to Maxim, who helped me figure out how to trigger the cheat mode.
Cheat mode Hold Reset and press pause 10 times. You get extra debug options on the main menu (for testing certain screens) and extra options in the Option screen for a level select, invincibility/infinite lives, and a boss-only mode.
Comfort difficulty In the Option screen, set the rank to "1 CRUSH" and then hold any button and press left.
Secret images Each time you complete the game, a new image appears on the Option screen, depending on the difficulty level you chose (including Comfort). There is also an image if you complete the game with the cheats on (making 7 images in total).