Site image is concerned with the images of your website. Here you will find a lazy load image setting. This lazy load image refers to the loading of images on your website after your other content is fully loaded. It means when visitors scroll downs on images then only images will be loaded and visible. The benefit of lazy load image is it makes your website content visible faster and makes your website load time faster.
Click on Dashboard>Appearance> Customize> Theme Customization> Site Image
By default lazyload image feature is disabled. You can enable the feature if you want to speed up your web page by slowing the load time of images of your website.