Hello, I have a problem with an album cover for an mp3 which started being used by other mp3's and I'd like to remove it. After I've selected file/add directory and the mp3's are displayed I right click on any mp3 and am not seeing any cover related options. Here is a screenshot of the right click menu. 
You'll find the cover-related options when right-clicking on the cover art window of the Tag Panel.
While you are in that menu you could investigate the hidden treasures of "Extended tags ...".