When you create an image to upload its important that the image be cropped to a 16:9 aspect ratio (1280x720). When you follow this sizing guidelines, Wordpress will automatically create multiple images at smaller sized to use throughout the site. This is important to optimize for mobile and speed up the size.
IMPORTANT: when you create and upload an image, make sure you optimize the size below 100KB. The smaller the file size the better. find the lowest optimization option to a desired visual quality. Failure to do so, will result in a slower page load and CWV score.