function.lazyload.php

<?php
function lazyloadImages($html) {
	$matches = array();
	
	preg_match_all( '/<img[\s\r\n]+.*?>/is', $html, $matches );
	
	$search = array();
	$replace = array();

	foreach ( $matches[0] as $imgHTML ) {
		
		// don't do the replacement if the image is a data-uri
		if ( ! preg_match( "/src=['\"]data:image/is", $imgHTML ) ) {
			$placeholder_url_used = 'data:image/gif;base64,R0lGODlhAQABAIAAAMLCwgAAACH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==';
	
			if( preg_match( '/class=["\'].*?wp-image-([0-9]*)/is', $imgHTML, $id_matches ) ) {
				$img_id = intval($id_matches[1]);
				
			}

			// replace the src and add the data-src attribute
			$replaceHTML = preg_replace( '/<img(.*?)src=/is', '<img$1src="' . esc_attr( $placeholder_url_used ) . '" data-lazy-type="image" data-lazy-src=', $imgHTML );
			
			// also replace the srcset (responsive images)
			$replaceHTML = str_replace( 'srcset', 'data-lazy-srcset', $replaceHTML );
			// replace sizes to avoid w3c errors for missing srcset
			$replaceHTML = str_replace( 'sizes', 'data-lazy-sizes', $replaceHTML );
			
			// add the lazy class to the img element
			if ( preg_match( '/class=["\']/i', $replaceHTML ) ) {
				$replaceHTML = preg_replace( '/class=(["\'])(.*?)["\']/is', 'class=$1lazy lazy-hidden $2$1', $replaceHTML );
			} else {
				$replaceHTML = preg_replace( '/<img/is', '<img class="lazy lazy-hidden"', $replaceHTML );
			}
			
			$replaceHTML .= '<noscript>' . $imgHTML . '</noscript>';
			
			array_push( $search, $imgHTML );
			array_push( $replace, $replaceHTML );
		}
	}

	$html = str_replace( $search, $replace, $html );

	return $html;
}

add_filter('the_content', 'lazyloadImages' );
add_filter('post_thumbnail_html','lazyloadImages');