If there is *any* kind of pattern to the thumbnails / full-size images, you can exclude them using output filter:
http://www.microsystools.com/products/sitemap-generator/help/website-crawler-output-filters/
I will be happy to help if you post your website URL. If you wish to have it remain private, email me the address!
http://www.microsystools.com/home/contact.php

In general I doubt it will matter much... But if you want Google to focus on certain images it makes good sense to only include those in the image sitemap I think
