The poster is missing or the movie matches the wrong poster.
The movie name is not correct and cannot be recognized, or it is matched incorrectly.
Select Unknow and the movie with wrong poster.
Press Menu on a remote control or long press left button on a mouse. And then select a poster
Press OK on a remote control and you can edit the name. Select Search and find the correct one.
Back to Movie Library and you can see the right poster.
Check out this video to finish setting.