MB uses two kinds of artist pictures - artist backdrops and thumbnails - in different panels. The artist picture panel uses backdrops, which track information view in now playing bar also shows on the right. It's also used by artist view of now playing tab, compact player, and gallery element of TM views.
What this TM view shows is artist thumb. If you use thumbnail browser or artists view on the main panel, you will see the same picture for each artist.
You can override the picture with your own using paste artwork context menu with the mouse hovering over the thumb either in thumbnail browser or main panel.