When you edit all files, the edits are saved to all files. Thus if you add an Artwork, it will be applied to all selected files.

You'll need to edit 1 Album add a time when manually adding Artwork.

I am scratching my head on how to do this or if it can be done. I want to add an image to the artwork of every music file in my library. but if I select my library then try to add the image it wants to merge all the artwork of every file into every file.
Does any one know how I can just add the same image to every file without merging every files artwork? I know it can be done one album at a time but WOW that is a slow process?