Function to add a class to all your "POST" images in WordPress and get them ready for lazyload.

functions.php

<?php
function lazyload_filter_content($content) {

	global $post; // getting The whole post objects 
	$content = $post->post_content;

	if( is_singular() && is_main_query() ) {

		//Renaming images for lazy loader!!!

        // Replace all image tags with the appropriately formatted HTML
        // We add a data-unveil attribute so we can target a CSS transition (fade-in)
        $content = preg_replace( '#<img([^>]+?)src=[\'"]?([^\'"\s>]+)[\'"]?([^>]*)>#', 
            	   sprintf( '<img${1}src="%s" data-src="${2}"${3} class="unveil"><noscript><img${1}src="${2}"${3}></noscript>', $placeholder ), $content );}

	return $content; // returning the edited $content
}

add_filter('the_content', 'lazyload_filter_content');