Just tried that but the Doctor Who image still comes up. I hadn't noticed these extra options before (just installed the latest version yesterday). Strange thing though... the Dr Who thing only seems to be the C:\Torrents\Completed folder, but not it's subfolders. For example the Wonder Woman animated movie downloaded in there, but in it's own subfolder and the thumbnail appears correctly.
EDIT: I found the damn Dr Who image. The fact it was only happening in that one folder made me suspicious so I set Windows Explorer to show hidden files, but nothing, then I set to show protected system files, and yup. Two images appeared. Both of the Doctor Who image called AlbumArtSmall.jpg and Folder.jpg. I've deleted those and finally the image is gone. The thumbnails in that folder are now movie reel segment icon than the folders all seem to have to which is a bit confusing but least the misleading Doctor Who pic is gone for good.
Also in there was thumbs.db which seems to appear in all folders I've browsed with Media Server. Is that it's thumbnail thing then?
EDIT2: Tinkered a bit more and it seems to be showing correct thumbnails for that folder now. I guess it just needed the Doctor Who image to be gone before it could fix things. I still don't get how this even happened but it's sorted now. Cheers.