Also, on the filetype: .pbm files are actually just .bmp files with the file extension edited. You'll have to convert it yourself, since my image host (Photobucket) converts .bmps into .jpgs. It's pretty easy, just open up the file in any editor, go to 'save as', choose the .bmp filetype, and save.
One more thing. Pixel put a little copyright tag inside all the .pbm files, and set it so the game won't recognize it without the copyright. You can deactivate the need for the tag using Cave Editor.