CD art on media folder

I have a 2 TB HDD via UPnP connected to my Raspi running OSMC.
This HDD was hitherto connected to a PC running iTunes.
There is one thing I’m missing.
When I scroll down the folders and files only the files display the cd-art, provided I get deep enough in the hierarchy of the folder pane. Most folders only display the standard folder picture. Some of the folders though display the cd-art as well. I cannot figure out the difference. I know that the cd-art is present, but I don’t know where and I seem not to discover a lead.
I’m quit optimistic that this (the display of cd-art) will be possible. I appreciate all the help I can possibly get.
Thank you

Thank you, Rondan.
I had read this quick start guide before.
The music is all there - presumably with all tags.

How did you add your music library?

I activated (and still activate) the iTunes library via UPmP

I checked again. My previous answer was probably a bit short and ambigous.
After activating the HDD with the iTunes library on it via UPnP I locate the files I want to play via Music / files / HHD name / (and now follows the windows file structure with -User / user name / Music (comes from windows) / iTunesMusic / iZunes Media / Music
and there we are. Finally the folder with the music. Very few folders display cd-art at this stage - though some do. When I open the folder I see the files and they all show the cd-art.
What I want is that the folder themselve display the cd-art.
At the moment I browse and study the Kodi documentation. But I don’t know the relation between Kodi and OSMC. As you will notice, I’m a bloody novice.
Thanks for interest.