Which backend extension are you using? Each extension has to implement images itself, so each one works differently, and some don’t implement it. Mopidy-File doesn’t currently support images, for example.
but unfortunately that didn’t make any difference.
From what you explained, I suppose this is because the playlists are using file:// URIs, which don’t provide images. Would switching them to local: URIs (so that Mopidy-Local-Images can handle the artwork) be the right way to get thumbnails showing in Iris?
I think Mopidy-Local is the only one of your backends that currently supports images. It doesn’t look like Mopidy-m3u does, and there’s been a pull request open for a few years to add images to Mopidy-File.
Probably a similar thing would have to be done for Mopidy-m3u.