README images are the one asset class you cannot quietly fix later. GitHub stores every version of every file in history, so a 6 MB screenshot you replace next week still costs every future clone, and the images render at whatever weight you committed forever. The numbers worth knowing are GitHub's own: a file added through the browser is limited to 25 MiB, the command line allows up to 100 MiB, git warns once a file passes 50 MiB, and GitHub blocks anything over 100 MiB outright — at which point you are pushed into Git LFS or a rewrite of history to get the blob back out. None of those ceilings matter for a README if the file is 150-400 KB, which is what a retina screenshot becomes after a resize plus a decent re-encode. Two more things GitHub does that surprise people: README images are served through its camo image proxy, so replacing a file at the same URL can keep showing the old cached version until the URL changes, and animated GIFs you commit stay animated but keep their full weight forever. Skwish resizes and compresses PNG, JPG and WebP in the browser with a per-file before/after readout and a ZIP download for a batch, so you can see the real numbers before you commit. Nothing leaves your machine, which matters when the screenshots are of unreleased work.