<table width="100%" border=1>
  <tr>
    <?php
    $array = array(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12);

    $count = 0;
    $bilderProReihe = 5;
    foreach($array as $element) { // oder while
      if($count < $bilderProReihe) {
        $count++;
      } else {
        echo '</tr><tr>';
        $count = 1;
      }
      echo '<td width="'.(100 / $bilderProReihe).'%">'.$element.'</td>';
    }
    $remainingCells = $bilderProReihe - $count;
    echo '<td colspan="'.$remainingCells.'">&nbsp;</td>';
    ?>
  </tr>
</table>