I have movie thumbs together with movies in one folder. Thumbs are named the same as movies only extension is changed to jpg. This fact prevents some clients to start the movie and the movie size show by the client is zero. If no thumb exist then the movie starts to play. At least xbmc and my blu-ray player dlna clients are affected. The wdtv live client works on the other hand.
I'm using minidlna 1.0.22