wrapping post images in div

I’m trying to wrap any image included in a Post witin a div with a given class in order to style. I’ve included this within my functions.php file:

<?php

if(is_admin()){

  add_filter('image_send_to_editor', 'wrap_my_div', 10, 8);

  function wrap_my_div($html, $id, $caption, $title, $align, $url, $alt){
    return '<div class="mydiv" id="mydiv-'.$id.'">'.$html.'</div>';
  }
}

?>

…but it doesn’t seem to work. I’m creating my own theme from scratch, and my php chops are minimal.

What am I doing wrong? Any help is apprectiated. A developent site for the posts page in question is:

https://meta.uoregon.edu/blog/?page_id=36