Create a traspanrant PNG file with your water mark on using your original thumb sizes
Code:
<img style="background:url(http://www.url-to-real-thumb.com)" src="http://www.url-to-transparant-png.com">
For TGPX:
Code:
<a href="{$gallery.gallery_url|htmlspecialchars}">
<img style="background:url({$gallery.preview_url|htmlspecialchars})" src="http://www.path.to/the/transparant/filemap/{$gallery.format|htmlspecialchars}.png">
</a>
Easy as that
