I took two ribbon thumbnail images after PowerGallery created them to try out your suggestion. The thumbnail image names ended with _62c where 62 was the default height of the ribbon thumbnails. I downloaded them and resized them to 100px square from 62px square and then replaced them via FTP.
Unfortunately, there was no apparent change in their size. I made sure to clear the cache, etc. It seems to me that there must be other files that restrict or affect their sizes when viewed in the ribbon panel. Do you have any other suggestions?