You have eventually added pictures to your whole collection if you go album by album - this is the function already available in MP3tag.
The way it is implemented at the moment lets you pinpoint exactly that release that you really have in front of you and you do not have to rely on some kind of automatism with its own criteria what would be the best hit.
If you do not really care then 3rd party programs like albumartdownloader can be integrated as a tool.