When you upload an image, WordPress does a lot of work behind-the-scenes so you can serve the image to your visitors. You can piggyback on this process and add custom image sizes to save yourself from having to resize images manually.

In this post, I’ll explain how and why WordPress creates different image sizes. Then I’ll teach you how to add your own image sizes by modifying the functions.php file, regenerate thumbnails and add your new image sizes to the Gutenberg image block and/or the WordPress loop.

WordPress Image Sizes Explained

Every time you upload an image into the WordPress media library, these are the additional image size options WordPress creates by default:

Read the entire post on WPMU Dev's site