I guess you could do that, but I don't know how to go about doing that. I tried editing the .pbm files in the data folder, but I couldn't run the game after.
That's because of the fact that all the Cave Story image files have a little hex string at the end which spells "(C)Pixel" to indicate that it's a legitimate Cave Story image, and if it doesn't, the game throws an error and exits out like you were experiencing. Back in the old days,
people used to manually edit images on an individual basis and add this "(C)Pixel" string to all edited images, or they would find the "(C)Pixel" string in the exe and remove it, which would remove the requirement for images, so that you don't have to go into a hex editor and add it every time you make a change to an image. Eventually, editors came around that just automatically remove the requirement for you so that you don't have to worry about this anymore. Cave Editor has a simple check box that does it, and Booster's Lab just automatically does it behind the scenes when you open up a game.