LOL. thanks mate. I did this myself a matter of months ago, but could never be bothered to fix it myself. thankfully you've spurred me into it so I no longer have an NCIS banner for my DVD Drive.
If I understand correctly, XBMC caches its thumbnails using a CRC of the source path. So, assuming I have that correct, and the CRC is always calculated the same (i.e. not CRC32 in some cases), and the DVD drive is always D, then to remove the thumbnail you'll need to FTP or Filemanager across to /Q/UserData/Thumbnails/Video/a/ and delete "a399b4ee.tbn".
Took me about 5 mins using a registry edit, and my thumbnails database FTPed to PC. Hopefully I followed my line of though correctly, and that should solve it for you too. (IMG:
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