It appears there is a bug (or new feature) in the latest version of iTunes (12.5) and iOS 10 where a majority of artist icons/thumbnails/images for missing when in the Library > Artists view, as per the screenshots below - they only show a generic microphone icon/thumbnail.
I haven't been able to find a setting/option to fix this at the moment. After doing some research I'm certainly not the first person to notice this "bug".
Only "workaround" for the issue currently is to not use the artist view and instead use the "Albums" or "Songs" view which show album artwork correctly and do not utilize artist images.
Let me know in the comments if you've noticed this bug as well, or if you have found a solution. I'll continue searching and will keep this posted updated with any fixes I find.
Update: I've found an apple discussion thread that states that the artist photo only shows up if you've purchased an album from that artist from the Apple Music store.