Cover in artist section

I use Volumio for some time and enjoy using the albums and artist section to choose. Using the album section is easy using a cover.png file in every folder.

My problem is the artist section where in most cases there is a fine cover been shown. But I have some additional folders named “audio plays” or “reading” or “language courses”. Furthermore I have some not so well known artists getting no covers on it’s own. Trying to put in some cover.png into the artists folder did not help. Prhaps someone knows a quick solution.

Kind regards,