TGPX can automaticly watermark galleries. But I know a way to easily overlay a watermork ontop of existing pictures. It does not really "watermark" the original pictures, it will be visible but not glued to the actual picture.
So if you're comfortable with overlay watermarks and would like to implement it, then hit me up on ICQ ( 565-013-936 ).
If you do not want to use overlays, then either find software that can watermark automaticly with smart thumbs (which i doubt that such software exists) or either step over from Smart Thumbs to
TGPX. They're running a huge discount for the last couple of days ;)