One compression, that is even better than Zopfli/Brotli/WebP, is FLIF (free lossless image format, see http://www.flif.info).
E.g. the webcomic image is reduced (lossless) to 320k. And what I found far more sophisticated: You can make the file as small as you want. If you “cut” the file, and just use the “start” of it, it is a progressive lossy encoding.
It also performs on different kind of graphic corpi (e.g. medical, geographical, photo, sketches, …) much more homogeneous regarding compression, opposing to many other algorithms which only shine with one kind of images, and perform not so good on the others.
Only drawback => its not yet arrived in any product, so its not useable. out of the box.