What is ICNS?
ICNS is the macOS icon container - what sits inside an .app bundle. Since Lion it stores its images as ordinary PNGs at a handful of sizes, which is why reading one means picking the biggest member and writing one means rendering several.
What is PPM?
PPM is a Netpbm image: a tiny text header giving the dimensions, then raw pixel values with no compression whatsoever. Almost nothing displays it, which is rather the point - it is what image-processing code and academic tools emit when they want to be trivially readable.