I don't really know how you would do it, and don't really see a need to. I believe that google base (formerly froogle) makes its own thumbnails from your full size image. I'm not sure what it would do with a thumbnail instead.
Stephen at Telafirma told me that he was not going to work on a version for 4.1.x xcart until it was stable as one of the issues has to do with thumbnail generation. There is a thread at
http://forum.x-cart.com/showthread.php?t=22308 that deals with this subject for version 4.1.x, you might check there.