So https://gtmetrix.com/ will show me images on my Wordpress website that are a large resolution and are being resized via CSS, eg:
XYZ.jpg is resized in HTML or CSS from 4207x3665 to 263x229. Serving a scaled image could save 983.3KiB (99% reduction).
At the moment I'm manually scaling down the images. My question is how I can make Wordpress do this automatically. My understanding was that it actually does this already - eg when you choose to display an image at the 150x150 size it creates a copy of the image and sizes it down to that. So how do I set this up on every image on my website?