from a purist's point of view this goes only half the way. You would also need the album name to identify the real cover. Do you have the single, ep, original album, greatest hits, best of, a sampler ..?
Anyway. MP3tag and the web source scripts rely on the album data and if that is not present, you would have to treat each file individually.
There are third party programs that might give you results with a workflow that you prefer, e.g. see here: