It looks like the poster in the link I sent took the still from the first frame or clip in the video. I suppose you could insert a different still if you wanted to.
Unfortunately, iMovie selects a random frame from the midpoint of the movie to set as the thumbnail. So you could change the midpoint by inserting a black background of suitable duration {EDIT} at the end of the movie. That would at least insert a thumbnail frame other than a black thumbnail. However, it still would be random, and not necessarily the specific still that you wanted.
If your movie starts with a title (thus a black fade) you might try trimming off the first few frames, or moving the title farther down the timeline, and see if that makes a difference when you share out.
Many people have posted here wanting to change or select the display thumbnail. Hopefully, in a future iteration Apple will add a feature that will allow selecting the thumbnail.
-- Rich