Samsung tablet Android 5.1.1. .thumbnails folder shows as .98gb so I run ccleaner (or Norton clean). Both show as DELETED yet, not much increase in memory. Reboot new .98gb file. I read that creating a .thumbnails FOLDER would prevent that - nope.
Afaik, thumbnails are instantly recreated for existing pictures; my guess is that it's a cleaner to get rid of thumbnails from/of no-longer-existing pictures.