I have used the Cleanup android app and other cleaning apps for a few years yet I have yet to find one that actually removes or reduces the size of the Thumbnails folder. This has happened across several Android devices.
I have tried deleting all my photos, emptying he Gallery and viewing the files over USB on PC, some guides say to create a new folder with the same name or setting it to Read Only. No luck.
I have tried with full Root/Superuser/Administrator privileges.
Has anyone found a method that works? The Thumbnails folder is 2.1gb and growing as time goes on.