Yeah, your PC probably can't see them because of the hidden file status.
You can delete the .thumbnails folder on the phone, but it will be recreated and then grow again.
To end this problem, you must delete the FOLDER called .thumbnails, and then create a FILE called .thumbnails.
(Be sure that this new file has the exact same name and is in the same place as the old folder.)
The dot before the name is important, as is the creation of the FILE instead of a FOLDER.
Android will not allow two things of the same name to exist in the same place.
The device wants to build a folder called .thumbnails, but it is not there and a file by that name already exists.
So in this way a new folder is blocked from being recreated, and because it is a file instead of a folder, it will not be added to by the app using it.
One issue with this is that cleaning apps often will clear thumbnails, and they will also clear your file.
So then the device will once again build a thumbnails folder, and it will grow.
Either don't use cleaning apps, or use one that allows you to add things to a whitelist- then add your file to that whitelist.