TIL I learned about a really useful, open source, image optimization app called ImageOptim. This app will help you batch resize image files (png, jpg, gif) in either lossless or lossy way.
It is so much quicker than using an image editor such as Affinity Photo. And best of all it's free!
It has helped me cut image sizes by as much as 94%! All without noticeable compression artifacts.
To install it on macOS simply run:
brew cask install imageoptim
For source and more info: