When sharing a playlist, the default image used seems to be the album art for the first song in the playlist. As a for-example, here is the default for a 34-hour ragtime playlist. The first song is Arlo Guthrie's
Ring Around the Rosy Rag. Therefore, the image used for the shared playlist is the cover art to the album
Alice's Restaurant, which while technically correct, is somewhat misleading. For instance, here is the link graphic to a shared playlist named "Ragtime, &tc." IMO the art should be more representative, and the link text should most certainly be the name of the playlist, NOT the name of the album of the first song in the playlist:

Ideally it seems that the default image for a playlist could either be a 2x2 of four different random album covers from the playlist, or perhaps a user-selectable group of four.
In any case, defaulting to just one album in the playlist for the playlist art is problematic.