I may have found a possible solution (well it worked for some of my missing artist photos in iTunes). I noticed that some of the artists with missing photos had an extra space after their name in iTunes.
So if you go into iTunes -
go to 'artists' under 'library' on the main page -
right click on an artist with a missing photo -
click 'artist info' -
look in the 'album artist' field -
see if there's an extra space at the end of their name -
if so, remove it and click 'ok'.
This has worked for me for most of the mainstream artists in my library, so has got rid of a fair number of those ugly microphone images.
Another point I've noticed is that changing the artist name slightly can determine whether iTunes will display a photo or not. For example 'The Bee Gees' in my library had a no photo even after removing the space. As this is a very mainstream artist, I thought to change the name album artist name to 'Bee Gees' and it worked bringing up an artist photo.
Good luck!