@normanb
The cause of this issue is that Strawberry does not find hidden album art in the album folder but amarok and clementine do.

I have taken the Clementine patch issued in 2012 and modified it for the current gentoo release of Strawberry and raised it as an issue on the strawberry github. https://github.com/strawberrymusicplayer/strawberry/issues/1809

Clementine-->Strawberry migration

I applied https://wiki.strawberrymusicplayer.org/wiki/Import_collection_library_and_playlists_from_Clementine
and initially everything looked fine. The covers showed up as expected in the Cover Manager and the play lists seemed to have transferred as well. However when I played a playlist the cover was 'blank' even though the cover was shown correctly in the 'Cover Manager' This applies to all Playlists. The same applies to any newly created playlist. Any suggestions as to how to investigate and fix this would be welcome.