Only require comment approval when an image is attached

You can create a module to do this by using the hook_comment_presave hook. If your module name is playx, then try this code:

function playx_comment_presave($comment) {
  if(strstr($comment->comment_body[$comment->language][0]['value'], '<img')){
   $comment->status = 0;
 }

I got this working using the Mollom module.

In Mollom's configuration, you can enable spam filtering and set up your own blacklist terms for the spam filter. I created blacklist terms for <img and "media-image" (TinyMCE's encoded image class). Then I told Mollom to "Retain the post for manual moderation" if it gets detected as spam.