Emacs has a lot of little known built-in features that can cover a lot of use cases provided by packages. This article presented some of them. Really nice to learn about them.

Also read the sequel, "More batteries included with emacs" (https://karthinks.com/software/more-batteries-included-with-emacs/).

ImageOptim equivalent on Linux.

Command line version: https://rubenerd.com/losslessly-optimise-images/