Yeah, the .pbm files in the Cave Story directory are basically .bmp files, except they have their file extension changed to .pbm. If all you want to do is just open up and edit the images, all you technically need to do is change the file extension to .bmp and edit it in paint or something like that.
If you tried to edit these images and then run Cave Story with them edited, then you probably ran into one of the complications that Serri alluded to. 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. 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.