I have noticed the same issue. My own suspicion is that Apple decided to let the publisher select the image to appear for the artist icon. I noted that the artists with a microphone image also have no image in the store page for the artist. It is just a guess, but my thinking is that the publisher is responsible for providing a licensed image and maybe the editorial description as well.
Totally agree that it is a pain. Artwork might be editable, but it would require a bit editor, because the artwork files are png images with a short header at the start. However, examining the xml database does not reveal any information for artists, just tracks--so the organization and location of artist artwork remains opaque. They might well only exist in a cache.
For those on the quest, I have found the following.
1. at ~/Library/Caches/com.apple.itunes/ is a file named ArtistNames.plist. I contains a listing of artist names as keys and a string of numbers as the value. The list contains many names that do not appear in my library, but NOT THE NAMES OF ARTISTS WHO HAVE A GENERIC ICON.
I suspected that this list provides a lookup into another file in the same directory, Cache.db. But that file was empty. Possibly, it identifies an artwork file
Similarly, at /Library/Frameworks/iTunesLibrary.framework/Versions/A/Headers is ITLibArtist.h, a header file for the ITLIbArtist class. This has no reference to an image. It just associates an artist with a number.
If one REALLY HATES the microphone, the file being isued is at /Applications/itunes/contents/Resources.MissingArtworkArtist.png
I suppose you could name another png file to that and replace the original.