Simply because xcart have called their database table "xcart_thumbnails" doesnt mean you actually need to include thumbnail sized images rather than larger ones. This mod stores large images in the "xcart_thumbnails" table (rather than in the detailed images table). Here is an example of the mod in use
www.lastangelsonearth.co.uk/heavenly/product.php?productid=116176
There is no need to have a popup with "large/detailed" images in it since we are already showing the large images inline within the product display page. (it would be entirely possible to use this mod without having ANY images in the "detailed" images table. I happen to include them since it means I dont have to add a "detailed" image to the "xcart_thumbnail" table for products which only have a single option -- ie my detailed image is a "default" image if there are no "variant" images available)
Personally I am using the "detailed" images table to store "detailed" images -- if you rest your cursor on the little magnifying glass in the previous url link, you will see a "detailed" image.
However, if you wanted to have "even larger" popup images (which are stored in the "detailed" images table) then you will need to do a bit of javascript programming to select the correct image for the popup.