Is not it possible to show the right cover for every MP3 file? Each MP3 has a different cover in the Explorer, but in Volumio the same is displayed for each file 
I may be wrong, as I’ve been trouble shooting my own tangle of album art today but…
In the Album folder there is probably a cover.jpg or folder.jpg (what I have) this is the artwork for that album that I believe Volumio uses first before any MP3 (or other) file artwork.
Delete it.
Re scan/update the folder. May need a browser refresh. CTRL+F5 (or other options)
Also, check you havn’t any recycle bins on the disk. Or hidden files within the same folder. These two items have caused most of my frustrations.
Hope this helps.
Wheaty
No, there is no xxxx.jpeg in the folder. Have already looked after it
New thought.
I have been using MP3Tag in windows to fix the minor errors I have. I had a duplicate artist that’s driven me nuts, only just realise I have an “of” and “Of” difference in the band name Argh!
Anyway, In Mp3 tag you can set both Front & Back covers etc… Could be that you have multiple covers set?
I have no idea on the organisation of this, but I did have two covers set on a couple of band demo albums, with confusing results.
Hope this helps.
Wheaty
no, it’s just always a front cover in the mp3 file