Does anyone know how to batch resize images?

Imagick's my go to... here's an article I wrote about it -- https://alligator.io/workflow/resizing-images-with-imagemagick/ -- ton of additional options as well, pretty much can do anything you need it to

irfanview

My favorite (and most performant) online image compressor is https://tinypng.com

They have a Web interface, an API and a WordPress plugin for automation.