Small optimization of your code

I have a site with many thousands of small images however your plugin is slowing the site down.
Changing your script to load the site_url into a variable halved the processing time of your plugin (added first line, used variable instead of function)

  $site_url = get_site_url();
  foreach ($html->find('img') as $img) {
    foreach ($rules as $rule) {
      if (strpos(str_replace($site_url, '', $img->src), $rule) === 0) {
        $img->class .= ' webpexpress-processed';
      }
    }
  }