Hook to use lazysize lib

inside-loop.php

// The lazyoad class is added

<?php the_post_thumbnail('medium', array('class' => "img-responsive lazyload")); ?>

lazyloading_thumb.php

<?php

//
//  If LazyLoad class is present on the postthubmnail we replace the html
//  markup to fit the lazysizes html markup plugin requirements
//

add_filter('post_thumbnail_html','lazyloading_thumb',10,5);

function lazyloading_thumb ( $html, $post_id, $post_thumbnail_id, $size, $attr ) {

  if (strpos($attr['class'], 'lazyload') !== false):
    $html = str_replace("", 'data-sizes="auto"', $html);
    $html = str_replace("sizes", "data-sizes", $html);
    $html = str_replace("srcset", "data-srcset", $html);
    $html = str_replace("src", "data-src", $html);
  endif;

  return $html;
}

scripts.php

<?php 


add_action( 'wp_enqueue_scripts', 'register_my_scripts' );

function register_my_scripts() {
  // get le librairie and save it locally https://github.com/aFarkas/lazysizes/blob/gh-pages/lazysizes.min.js
  wp_enqueue_script( 'lazysizes', get_template_directory_uri() . '/js/libs/lazysizes.min.js',false, '3.0', true );
 }