Tried to find a solution in the forum.
My music is mainly jazz concert recordings in FLAC, most without
proper album art.
I play my music through a wireless Squeezebox that also displays the cover
of the Album, on the Squeezebox SlimServer on my computer screen.
I also use milka's wonderful scheme to print covers, including Album art.
Therefore I am adding a file "cover.jpg" to each folder, with artist photo.
Is there a batch action that can take the a cover.jpg from my harddisk and
add it to each of the Miles Davis folders that I have open in MP3tag (if there isn't a
cover.jpg there in the folder already)?
I think that's not possible.
Maybe this is not so elegant, but you can try to create an export configuration file which creates a batch file that you can run from the command prompt.
Otherwise you may try to create a batch file that actually does the work for a single cover, something like:
copy \covers\%1\%2\folder.jpg %3\folder.jpg
and save it as CopyCover.bat somewhere on your HD.
You may then setup a new tool menu item (see Options\Tools dialog) to run this batch with the correct parameters, for the previous example "%artist%" "%album%" "%_folderpath%" (Don't forget the " character because the artist name or the album title may have some space inside).
To run this tool just right clic one file for album and choose your command from the tools menu.