Interesting observation... 🙂 I think there must be a separate online repository of "artist images" (different from album artwork) that Apple maintains on their servers. When I do a right-click on an artist that has an actual picture shown in iTunes Artists view, and select Go to -> Show in iTunes Store (or Show in Apple Music), that same artist image appears there. OR, they have a much fancier special presentation. When I do the same for an artist that has the generic "microphone" image, their presentation in iTunes Store is also "generic" without a separate artist image.
Therefore, if you see the microphone, it probably means that artist's artist image is currently not available.