nickweller sends a link to an informational post about FLIF, the Free, Lossless Image Format. It claims to outperform PNG, lossless WebP, and other popular formats on any kind of image. "On photographs, PNG performs poorly while WebP, BPG and JPEG 2000 compress well (see plot on the left). On medical images, PNG and WebP perform relatively poorly while BPG and JPEG 2000 work well (see middle plot). On geographical maps, BPG and JPEG 2000 perform (extremely) poorly while while PNG and WebP work well (see plot on the right). In each of these three examples, FLIF performs well — even better than any of the others." FLIF uses progressive decoding to provide fully-formed lossy images from partial downloads in bandwidth-constrained situations. Best of all, FLIF is free software, released under the GNU GPLv3.