hmmm.. what is the value of the image currently being displayed? Is $id a visitor id or the current image id?

assuming $id, $firstrec, & $lastrec are numerical codes representing current image, first image, and last image, and your image codes are sorted numerically.. maybe you're looking for something like this?

if ( $id < $lastrec ){
  print $q->start_form(-name=>'displaynext',-method=>'POST',-action=>'
+next.cgi'); 
  print "<input type=hidden name=i value='$id'>";
  print "<input type=hidden name=u value='$username'>";
  print $q->submit(-name=>'next',-value=>'next');

  print $q->end_form();
}
if ( $id > $firstrec ){
  print $q->start_form(-name=>'display prev',-method=>'POST',-action=>
+'prev.cgi');
  print "<input type=hidden name=i value='$id'>";
  print "<input type=hidden name=u value='$username'>";
  print $q->submit(-name=>'prev',-value=>'prev');
  print $q->end_form();
}
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