What is HEIC?
HEIC is what an iPhone saves photos as, and it is not really an image format so much as a container holding HEVC video frames - the same codec used for film, applied to one still picture. It stores a photo at roughly half the size of the equivalent JPEG, which is why Apple switched to it, and it is also why so little else opens it: HEVC is patent-licensed, so most browsers and a good deal of desktop software cannot read a HEIC at all. Large photos are usually stored cut into tiles that have to be reassembled, not as one picture.
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.