Based on the original code in Pager with thumbnails amended about thumbnails dimensions and CSS.

1. To file template.php add function:

   /* Image Gallery Pager with thumbnails */
   function custom_pager_thumbnails($current_nid, $class = NULL) {
     $tid = reset(array_keys(taxonomy_node_get_terms($current_nid)));
     $result = db_query(db_rewrite_sql('SELECT n.nid, n.title, f.filepath FROM {node} n INNER JOIN {term_node} tn INNER JOIN {files} f ON n.nid = tn.nid AND n.nid = f.nid WHERE tn.tid = %s AND n.status = 1 AND f.filename = \'%s\' ORDER BY n.sticky DESC, n.created DESC, n.nid DESC'), $tid, 'thumbnail');
     while ($node = db_fetch_object($result)) {
       $nodes[++$i] = $node;
       if ($node->nid == $current_nid) $x = $i;
       // image size
       $image_info = image_get_info($nodes[$i]->filepath);
       $image_width[$i] = $image_info['width'];
       $image_height[$i] = $image_info['height'];
     }
     if ($i < 6) {
       while (++$j <= $i) {
         $output .= l('<img src="' . check_url(url('system/files/' . $nodes[$j]->filepath)) . '" alt="'. check_plain($nodes[$j]->title) . '" width="'. $image_width[$j] .'" height="'. $image_height[$j] .'" />', 'node/'. $nodes[$j]->nid, array('title' => check_plain($nodes[$j]->title), 'class' => $class), NULL, NULL, FALSE, TRUE);
       }
     }
     else {
       switch ($x) {
         case 1:
           // first image
           $y = array($i-1, $i, 1, 2, 3);
           break;
         case 2:
           // second image
           $y = array($i, 1, 2, 3, 4);
           break;
         case $i-1:
           // penultimate image
           $y = array($x-2, $x-1, $x, $i, 1);
           break;
         case $i:
           // last image
           $y = array($x-2, $x-1, $x, 1, 2);
           break;
         default:
           $y = array($x-2, $x-1, $x, $x+1, $x+2);
       }
       foreach($y as $index => $z) {
         if ($index != 3) {
           $output .= l('<img src="' . check_url(url('system/files/' . $nodes[$z]->filepath)) . '" alt="'. check_plain($nodes[$z]->title) . '" width="'. $image_width[$z] .'" height="'. $image_height[$z] .'" />', 'node/'. $nodes[$z]->nid, array('title' => '('. ($z) .') '. check_plain($nodes[$z]->title), 'class' => $class), NULL, NULL, FALSE, TRUE);
         }
         else {
           $output .= '<span class="active" title="('. ($z) .') '. check_plain($nodes[$z]->title) .'"><img src="' . check_url(url('system/files/' . $nodes[$z]->filepath)) . '" alt="'. check_plain($nodes[$z]->title) . '" width="'. $image_width[$z] .'" height="'. $image_height[$z] .'" /></span>';
         }
       }
     }
     return $output;
   }

2. Add the following to node-image.tpl.php (a copy of node.tpl.php in your theme folder):

   <!-- Image Gallery Pager with thumbnails -->
   <div class="pager">
     <?php if ($page != 0 && $terms) { print custom_pager_thumbnails($node->nid); } ?>
   </div>

3. Add to your style.css e.g.:

   /* Theme snippet: Image Gallery Pager with thumbnails */
   .content .pager {
     margin-top: 3px;
     margin-bottom: 3px;
   }
   .content .pager img {
     margin-bottom: 3px;
     vertical-align: middle;
   }
   .content .pager .active img {
     border: 5px solid #f93;
     margin-bottom: 0;
   }