.Thumbnails are not real pictures, only extremely low resolution versions of your pics and videos for easier searching for you when using file managers, galleries, and other such apps.
You can copy them, but the quality will be disappointing.
These are designed to be viewed in miniature.
Your computer is probably having dificulty with it because it is a hidden file, as noted by the " . " in front of the title.
Anyway, go into your file manager, and copy the file to another location that you create.
Do not use a " . " in the name of the new folder, and so the new folder will not be hidden.