You can just add an IF statement around the code you want conditionally added like:
<?php if ($row_WADAproducts['Big_Pic_Name']) { ?>
<a href="products_detail.php?ID=<?php echo($row_WADAproducts['ID']); ?>
<?php } ?>
<?php echo(isset($_GET["pageNum_WADAproducts"])?"&pageNum_WADAproducts=".intval($_GET["pageNum_WADAproducts"]):""); ?>" >
<font color="#1c2372"><?php echo($row_WADAproducts['Product_Name']); ?></font>
<?php if ($row_WADAproducts['Big_Pic_Name']) { ?>
</a>
<?php } ?>