Volumio and Mpod/Mpad cover art


I feel really stupid, I followed the rather simple instructions for getting local cover art via Mpod/Mpad, but I get nothing.
My Volumio setup is the following:

volumio.local ( in my lan)

My NAS mount is NAS/MyMusic in Volumio and is located in the NAS at

I put the cover art file folder.jpg in each album directory

What should I write in Mpod in the field local cover art URL?

Thank you for your help