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.
What is PNG?
PNG is the web's lossless image format: compression that never degrades the picture, and a real alpha channel for transparency. It is the right choice for screenshots, logos, line art and anything with sharp edges or text.