How to change Artist profile image?
If you've added custom artwork to an Artist, it will replace the individual Album Artwork that iTunes had previously set.

To clear it, you need to clear each affected album - which can be done as a bulk selection, using shift/click to select all its tracks [but doesn't work if you select more than one Album]
then  


Cmd/I to Get Info  
Artwork Tab  
Select the Custom picture  
Delete or Backspace  
OK  
File Menu > Library > Get Album Artwork
You can do the last step once only, after resetting all the tracks you need to clear the Custom picture from.