Here's some additional information.

I copied 122 JPG's into the _Source directory, went into the Maintenance screen, selected all, and then synchronized.

The gallery showed nothing was in the album.  I then repeated the process with the same results.  Once I copied the files from the _Source directory to it's parent then the gallery showed the images. 

Approximately 40-50% of the images do not display thumbnails.  If I check the _Thumbs directory all 122 files have a thumbnail.  More than once I have cleared the _Thumbs folder and Synch'ed again, but the same files continue to not display their thumbnails properly.

However, as someone else here also noted, if I upload the file through the interface then one of the images which was previously without a displaying thumbnail now displays one.

There does appear to be something different in the handling of images that are placed into the directory manually vs those uploaded through the interface.