What is WEBP?
WebP is Google's image format for the web, and the rare one that does both jobs: lossy compression that beats JPEG at the same quality, lossless compression that beats PNG, and an alpha channel either way. Every current browser reads it; older software often still does not.
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.