you need to use an if statement to check if the column has a value, for example:
<td>
<?php if(($row_DetailRS1['photo6'] != NULL) && ($row_DetailRS1['photo6'] != "")) { ?>
<a href="<?php echo $row_DetailRS1['photo6']; ?>" target="_blank"><img src="<?php echo $row_DetailRS1['photo6']; ?>" width="100" height="100" border="0" /></a>
<?php } else { echo " "; } ?>
</td>