Changing thumbnail for iMovie project file
I am using a 2016 MacBook Pro and iMovie 10.1.9. I understand iMovie auto-selects a frame in the midpoint of the movie to display as a thumbnail. However, this particular frame in my movie is about the most lousy frame in the whole movie to present as a cover or representing the movie. I have it all fine tuned and don't want to mess with the content of the project. How can I choose a different frame to use as a thumbnail or cover image?
iMac Aluminum & Mac Book Pro, Mac OS X (10.5.2)