What is AVIF?
Built on the AV1 video codec, and the smallest of the widely-supported image formats by a clear margin - frequently half the size of a JPG that looks the same. It handles transparency and HDR. Being new is its only real drawback: plenty of software still won't open one.
What is PGM?
Greyscale, uncompressed, with a header you can read in a text editor. PGM is part of the netpbm family, and it exists because sometimes you want pixel values you can do arithmetic on without a decoder in the way. Image processing and computer vision code loves it.