This is an example of lazy image loading with responsive images and progressive JPEG image format.
These best practices about web performance were implemented using LazyLoad by Andrea Verlicchi.
Responsive images are implemented using the img tag with the srcset and sizes attributes, and picturefill as polyfill for older browsers.
Images are loaded only as they enter the viewport through the usage of my LazyLoad script, which supports the srcset attribute in addition to the good old src attribute.
Progressive JPEG rendering is guaranteed through the usage of the show_while_loading option of my LazyLoad script.