Q: lots of missing artist artwork in iTunes 12?

Hi how do I control the artwork in the Artists view of iTunes?  A large number of my artists have a generic microphone picture so the initial issue is how to get ANY artwork then, if this is automated, the second question is whether the picture selected can be changed.

All albums in my library have artwork.  I have seen some old posts stating that iTunes selects it based on the highest rated song/album but I do not use ratings and yet many artists have artwork, and when I change ratings and restart iTunes it has no effect; so I think this is how it used to work, but not so is later iTunes versions.

I am running iTunes 12.5.1.21

Here is a shot of showing example of what my library looks like: