Display a promo image at last in every slideshow created for a node
Create a Promo article and add the promo image as the field in this article. 

Note the this article will be just a placeholder for us and will not be presented anywhere. So may you could un-publish as well. 

Do a dpm to find out the fid or any related that you need to know about this particular image field. Then make changes to the view in post_execute.

   function modulename_views_post_execute(&amp;$view) {
      // wait for the right view
      if($view-&gt;name == "name_of_the_view") {
        // create the object you want to insert in the result
        // dpm($view-&gt;result); to check how the objects are built. 
        $temp = new stdClass;
        $promo_image-&gt;fid = 13;

        // replace the old result array
        $view-&gt;result[] = $promo_image;
      }
    }


Based on the answer : Insert row into a view