If the main image upload folder is 'web', and the thumbs upload folder is 'web/thumbs', and if you set the upload filename for both image and thumb to [FileName], then both image and thumb will have the same filename (i.e. the name of the original image). You would reference one (Large) from the 'web' folder and the other (thumb) from the 'web/thumbs' folder.
So in your scenario, to reference the large from the thumb, you would have:
<a href="images/portfolio/web/<?php echo $row_PortfolioImage['PortfolioImg']; ?>" title="Title goes here">
<img src="images/portfolio/web/thumbs/<?php echo $row_PortfolioImage['PortfolioImg']; ?>" rel="web" alt="alt goes here" title="title goes here"></a>
Both would be 'PortfolioImg'. You wouldn't need another column in the table, as the image just has one name.
Also completely misunderstood you last time, sorry about talking rubbish.