how to comment individual image in popup form drupal 7?
First, in your content type edit page, make the "Preview Before Submit" option disabled or optional. Once you do that you will see the Submit button. 

Even you make the Submit button appear, the same will happen and user will be redirected to the actual page with no modal display. From the pattern of your modal window, I guess you are using colorbox plugin. If yes then you need to open this colorbox as an iframe option.

&lt;script&gt;
    $(function () {
        $(".example6").colorbox({iframe:true, innerWidth:425, innerHeight:344});    
    })
&lt;/script&gt;


So even you submit the page, the page will not be redirected outside of modal. 

EDIT #1 
Then I think you may have to alter the form you are using and check for form elements with a debug() function. Then I guess in your form_alter you can do the following,

&lt;?php
  function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if (isset($form['type']) &amp;&amp; $form_id == 'mymodule_node_form') {
      // Hide the preview/submit button.
      $form['actions']['preview']['#access'] = FALSE;
  }
}
?&gt; 


Have a check on the variable that sets the preview, by 

variable_get('comment_preview_' . $node_type);


If it's set then can be disabled via,

variable_set('comment_preview_' . $node_type, DRUPAL_DISABLED);


I hope this will be helpful to your cause.