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.

Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a PGM file above and choose afterwards.