Add a span tag around image

try to add a filter function to functions.php of your theme;

example:

add_filter('the_content', 'alc_image_spanner');

  function alc_image_spanner($text) {
    $text = preg_replace_callback("#<img(.*?)>#is", create_function('$matches', 'return "<span><img".$matches[1]."></span>";'), $text);
    return $text;
  }