More details are needed: You are using Media Library? Where are your video files and how is the SMP accessing them?
If you are using ML, and have your files on a WIndows share on a PC, then you can certainly get the thumbs to update by: (1) powering down the SMP (pull power cord), (2) deleting the .wd_tv directories from all shares, (3) powering your SMP back up and letting it rebuild the ML and .wd_tv directories. I have to do this occasionally when I change thumbs and want the changes to appear immediately.
If you say the old thumbs are still being shown after doing the above, then something is very wrong. Where exactly could the old (supposedly now deleted from the shares) thumbs possibly be coming from? This implies to me that you really haven’t changed the thumb. How exactly are you changing a movie’s thumbnail? The thumbnail for Movie.mkv, for example, is Movie.metathumb.
