Move DCIM .thumbnails to external SD
Moving this folder to sd woun't help, cause android would recreate it. For me, this folder is shown as 6.5 GB (the real size was aprox 80 MB).

WorldCAD Access posted an interesting solution, how to prevent Android for recreating at least the db files.

Another soultion, if you have root access would be to symlink this folder to sdcard. You can do this via command line or using tools like Link2SD or FolderMount.

here you also can find some suggestions/solutions.