PPM
The easiest image format to parse by hand.
Where it came from
PPM (Portable Pixmap) is part of the Netpbm toolkit's "portable anymap" family from the late 1980s — PBM (1-bit), PGM (gray), PPM (color). The goal was interchange between Unix image tools, not compression.
Its charm is pedagogical: you can read and write a PPM with a few lines of C. It is the "hello world" of image formats.
How it works
A PPM is a magic number (P3 for ASCII, P6 for binary), then width, height, max-value, then raw pixel triples. No compression, no metadata — just numbers.
That verbosity is why it never hit the web, but it is perfect for pipelines and learning. UWC converts PPM → PNG/JPEG server-side.