Gallery Loop (attachment)

<div class="gallery-slider">
   <?php
      $attachments = get_posts( array(
         'post_type' => 'attachment',
         'post_mime_type' => 'image',
         'posts_per_page' => -1,
         'post_parent' => $post->ID

      ) );
      ?>
   <ul class="slides">
      <?php if ( $attachments ) {
         foreach ( $attachments as $attachment ) {

            $thumbimg = wp_get_attachment_image( $attachment->ID, 'exhibition_img', true );
            $thumbimg_url = wp_get_attachment_url( $attachment->ID, '', false );

         echo '<li><a href="'.$thumbimg_url.'">' . $thumbimg . '</a></li>';
         }



         }
         ?>
   </ul>
</div>