In index.php make the following changes:
Find:
$select_column_list .= 'p.products_image, ';
and add below:
$select_column_list .= 'p.large_products_image, ';
Also in index.php you will need to include the css code.
Do not perform this next step in a production environment.
In product_listing.php the following changes need to be performed:
Find:
$display .= '<table summary="" width="100%" ><tr><td style="text-align:center;" valign="top" height="'.PRODUCT_IMAGE_VSPACE.'" >'.$image.'</td></tr></table>';
and replace with:
$display .= '<table summary="" width="100%" ><tr><td style="text-align:center;" valign="top" height="'.PRODUCT_IMAGE_VSPACE.'" >'.$image. tep_image(DIR_WS_IMAGES . $listing['large_products_image'], $listing['products_name'], '', '', 'class="large"') .'</td></tr></table>';
You will need to add a link in there as well.
I can't really take this any further as I would be coding in the dark.