iMovie: Selecting Project Image
Trying to select a different image in the project menu
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Unfortunatly, no easy way to change the project icon thumbnail. iMovie randomly choses a frame at the midpoint of the project to set as the poster display on the project icon. You can change the midpoint by inserting a black background of suitable duration at the beginning and/or end of the timeline. Or you could change the clip that is the present centerpoint.
— Rich