Sometimes I have such trouble ! The cover of the track is not displayed.
I cast by using Bubbleupnp and Mconnect. At the beginning, pictures were displayed correctly. Since yesterday here is what I have!
It appears on Chrome/Windows or Anfroid Myvolumio apps.
When i listen music I have in the local library (on the sd cad of the the RPI), picture is ok.
Have I cache to clear ? A log file to read ?
Thanks for helping