Asset.php

function get_attachment_id_from_src ($src) {
  global $wpdb;
  $reg = "/(-e\d{13})?(-[0-9]+x[0-9]+)?\.(jpg|jpeg|png|gif)$/i";
  $src1 = preg_replace($reg,'',$src);
  if($src1 != $src){
      $ext = pathinfo($src, PATHINFO_EXTENSION);
      $src = $src1 . '.' .$ext;
  }
  $query = "SELECT ID FROM {$wpdb->posts} WHERE guid='$src'";
  $id = $wpdb->get_var($query);
  return $id;
}

$image_thumbnail_url = wp_get_attachment_image_src( get_attachment_id_from_src ($image_src), $thumbnail_name);

Little patch that allows to have modified images within wordpress to also work