Many people were having this problem. One possible fix comes from marloh. “Here is the solution: Play the movie in iTunes, pause it, slide the pointer in the time bar back to 0:00. This only works if the pointer is at several minutes or better somewhere in the middle of the movie. You may slide it to the right first and then back to zero. Then click on the x on the upper left corner to close the movie window. Add the cover pic you prefer and it will be saved. “