Copied the answer by Chris CA from Apple Support Communities thread.
To easily move some of your library...
Now you can delete the TV shows from /Music/iTunes/iTunes media/TV Shows/.
All metadata will be retained.
After you are finished, go to iTunes prefs Advanced and reset the iTunes media folder location to the location you noted in step 2.
The artwork or poster frame or default image (whatever you call it) of an iTunes video (especially if its made and published from iMovie) can be changed by the following steps
1. Double click and play the video in new window
2. Pause/Stop and drag the time scale to the frame you want to set as poster frame
3. Right click on the video and select "Set Poster Frame"