If Next Image

I’ve been working with WordPress’ built in gallery feature and have back/next buttons below the individual attachment pages/images, but the problem I am having is when I am at the first or last image in the gallery and there isn’t a previous or next image, the button disappears. I tried the code below, but I don’t think the get_next_post is right, it should be something else like if_there_is_another_attachment_in_your_wp_gallery_blah

<?php

    if ( get_next_post() ) {

       next_image_link( false, '<img src="/next.jpg">' );

   } else {

      next_image_link( false, '<img src="/next.jpg">' );

   };
?>