Hi Arthur,
I am using a raspberry pi 3 and the latest runeaudio version.
On a RP3 it should work as described in my last post.
Please check your files for embedded cover art.
The file search (cover.jpg) is case sensitive, maybe this is your issue.
I did some tests on my RP3 and all works like it should.
The same song with all possible cover locations.

- cover-test.jpg (152.21 KiB) Viewed 2568 times
I am very interested in why it doesn't work for you.
Use
WinSCP and
Notepad++ on your host PC to edit files.
If you are on a MAC you can use
Cyberduck instead.
Do not use built-in windows editors, because they may break file encodings and line endings.
Frank