how to set the poster frame

You might take a look at this link, where the poster found a way to force a poster thumbnail.

Forcing a cover shot in Imovie - Apple Community

— Rich

iMovie randomly sets the poster frame as that of the frame that appears in the middle of the movie. No real way to change it, other than to add a black background to the beginning or end of the move to change the center point, or perhaps rearranging the clips in the middle of the movie to display the frame that you want.

-- Rich

Yes, lots of folks have posted on here wishing for the same thing. Poster selection would be a nice feature for Apple to add.

-- Rich