What happens when you use:
<a href="images/portfolio/web/<?php echo $row_PortfolioImage['PortfolioImg']; ?>" title="Title goes here"><img src="images/portfolio/web/thumbs/<?php echo $row_PortfolioImage['Filename_Thumb']; ?>" rel="web" alt="alt goes here" title="title goes here"></a>
Does that not display the thumbnail?