Load images after the whole page has loaded by just adding the class .async-img

images.js

// Default image data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mO8dPv2fwAImAOJMh4kwgAAAABJRU5ErkJggg==

function load_images(){
    var images = document.querySelectorAll(".async-img");
    for(var id=0; id<images.length; id++){
        var image = images[id];
        var img_url = image.getAttribute('url');
        image.src = img_url;
    }
}