You can’t link the entire/slide image to an lightbox iframe popup. However you can select “Slide Link?” – “No Link” and then insert your iframe link into the slide title or description field (both fields support html code). The link must look like
<a href="http://www.google.com?iframe=true&width=100%&height=100%" rel="prettyPhoto[iframes]" title="Google.com opened at 100%">Google.com</a>
and replace http://www.google.com with your url and the title and link text with your custom text. Note that you can not use the slide itself as a link if you add a link to the title/description field . otherwise you would add a link to a link and that’s not a valid html code.