I have this need, that is, I have a folder that contains 1000 mp3s of Various Artists, each mp3 has the "artist" tag valorised with the correct value, what I try to do is import the artist's cover from online sites that already provide MP3Tag and that does so based on the tag value stored in the mp3.
I understand that this function is not really foreseen...
You are right. This function is not available in MP3tag at the moment.
You could have a look at albumartdownloader.