Thanks, but for consistency the intent was to populate the artist thumbnails from the embedded artwork, rather than to download whatever last.fm chooses to offer at the current time (or not, for the lesser-known artists). I've started doing this manually now, its more tedious than expected to get the exact file names, but it does work so I will continue if I decide to use MC more often
I happen to use Mp3tag for managing all my FLAC and MP3 tags at the moment, though others will do this too. As you mention, Artist art can be downloaded if they are popular enough, but personally I chose/edited/made my own images that best represents the artists as I see them (which for the longer running ones may not necessarily be as they are now).
For a while I've been using foobar2000 which will read/display (and i believe also manage) artist art - though I forget which UI elements I used to configure foobar with. However the artist images are just standard ID3 tags, like front cover art, and not exclusive to any particular player; some players just offer better support of ID3 than others.