I converted the pbm to a bmp, and then used Paint.NET. Also, on the PC, they are in the data folder in the Cave Story folder. Not sure about the Mac though.
You should probably look at the Jenka's Nightmare thread for questions like that, or just PM me. I'll give you a hint though. In Labyrinth B, look for the red blocks.
Actually, I am planning one, but I am not ready to give details yet. I'm sorry that I made us quit our Champix project, but I wasn't ready to do that yet.