Change Output for Images in Content

One way is to do this dynamically:

function do_the_replacements($matches){

  // $matches[0] = full string
  // $matches[1] = link attributes
  // $matches[2] = link contentes (the image)

  // change 'someclass' here...
  if(strpos($matches[2], 'someclass') !== false){
    return '
      <div class="featured-img">
        <a '.$matches[1].'>'.$matches[2].'</a>
        <div class="corner-nw"></div>
        <div class="corner-ne"></div>
        <div class="corner-sw"></div>
        <div class="corner-se"></div>
      </div>
    ';

  }

  // no matches, leave it as it is
  return $matches[0];
}

function my_content_filter($content){
  return
    preg_replace_callback('#\<a(.+?)\>(.+?)\<\/a\>#s', 'do_the_replacements', $content);
}

add_filter('the_content', 'my_content_filter');