In this article, we explain how to resize an image on a Mac, explore the importance of learning how to do it, provide tips, and describe how to crop images.
To change settings in the Photos app on your Mac, choose Photos > Settings, then click General, iCloud, or Shared Library at the top of the settings window.
I recently found ImageOptim for mac (or online) and it's changed my game. I will never upload another image online without first running it through this.