gimptool --install-bin recurtnJust install this plug-in and point it a directory and then go and take a nap or whatever. It will recursively go through any subdirectories and create thumbnails of images in them as well.
TODO:
Run it on its own and it removes any "failed" thumbnails. It also scans through all the current thumbnails and verifies that the original file still exists. If the file is gone, the thumbnail is removed. Also, you can pass it a string as an argument and any thumbnails whose original filename contains that string will be removed as well. Perfect for a weekly cron job.
Examples:
tnclean.pl > /dev/null
tnclean.pl digital/old