My original question is invalid.
YouTube allows users to set thumbnails of videos to be any properly formatted image. YouTube then only allows users to set a specific video thumbnail as the playlist thumbnail. It is not possible to set an image as the thumbnail of the playlist unless it is a thumbnail of a video in that playlist; and in that use case the video is associated to the playlist for the representative thumbnail.
Many answers about using the Youtube Data API v3 to get the thumbnail of a playlist; and many answers for how to get or set the thumbnail of a video.
But none about how to set the thumbnail of a playlist. The documentation shows nothing and no searches into the API documentation or Stack Overflow find the question or its answers.
I tried using the thumbnails().set() method for setting thumbnails to videos, but that returns a permission denied error indicating that using playlist IDs in its request for video IDs is not a good monkey patch.
Please help.
Q: set thumbnail of a playlist?