David,
use the following code to replace each of the instances of your thumbnail replacing $row_WADAinventory_tbl['Photo_1_Thumb'] with $row_WADAinventory_tbl['Photo_2_Thumb'] etc.
<?php echo $row_WADAinventory_tbl['Photo_1_Thumb']?'<a class="thumbnail" href="#thumb"><img src="' . $_SERVER['DOCUMENT_ROOT'] . '/Auto_Dealers/' . $row_WADAinventory_tbl["Dealer_Name"] . '/images/' . $row_WADAinventory_tbl["Inventory_ID"] . '/' . $row_WADAinventory_tbl['Photo_1_Thumb']; . '" border="0" /><span><img src="<?php $_SERVER['DOCUMENT_ROOT'] . '/Auto_Dealers/' . $row_WADAinventory_tbl["Dealer_Name"] . '/images/' . $row_WADAinventory_tbl["Inventory_ID"] . '/' . $row_WADAinventory_tbl['Photo_1_Resize']; . '" alt="" border="0" /></span></a>':""; ?>