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 PGM?
PGM is the greyscale member of the Netpbm family: a three-line header, then one brightness value per pixel with no compression. It is what image-processing courses, OCR pipelines and computer-vision tools pass around when they want a format with no decoder in the way.