The bad news is: the covers may be stacked in different orders. So there is no real "first" and "second" - you could try it like this:
Load the files
Select the files
Open the extended tags dialogue
Scroll through the embedded pictures and see if you get a display for the common one. If you don't then hard luck. If you do: delete the displayed picture with the red x next to it.
Here is a plan b that relies heavily on your thoroughness and that you have set at least one picture to "artist":
Load the files
Select the files
Create an action (quick) of the type "Export cover to file"
This should create 2 picture files for each audio file: one for the album, one for the artist.
Now you delete all embedded covers with the help of MP3tag.
Then you create an action of the type "import cover from file"
This should import the artist picture into the file.
There is no chance in Mp3tag today to remove individually some embedded cover image.
You can apply an action to remove all embedded cover images at once.
Afterwards you can embedd the artist image again, just from the folder, where the image file has been stored.
In case of there has been stored no image file on the disk, then at first you have to save the embedded images to disk folder, giving the images an attributed name, in order to be able to reload them automatically later on.
You can collect these steps within an actiongroup.
Apply a proper filename to store the images to disk, ... e. g. Formatstring: %path%''%_cover_type%
Remove all cover pictures from the file.
Reload the wanted cover image by applying the fitting formatstring as needed.
While searching, I found another way. There is a conversion software that did the trick. I has an option to 'keep only the front cover' when converting a file, (even if you convert it to the same format as it was, i.e. FLAC, i.e without modifying the original file). And it worked. I got all 500 files with the second cover art removed while keeping the first intact.
I just wanted to congratulate for a great piece of software! I donated as I really appreciate the work done, and how useful MP3TAG is.
Although I've been programming in assembler, fortran, pascal, basic and C many, many years ago, unfortunately I don't have the time and practice anymore to do it. But I understand some of the steps still involved in the process.
I succeeded in exporting the embedded front cover of tracks in the respective albumartist\album folder, using the 'export cover file' action, with the cover named according to the title %title%.
Then I batch-optimize the cover using a software called 'RIOT', reducing all sizes to 500x500px and optimizing at 75% jpg. Greatly reduces the size of all covers while keeping visual quality.
Now when I want to 'import cover file', it works individually when I go in each folder separately, but how can I have MP3TAG go automatically to the right folder to import each cover file? I assume somewhere I have to use the %path% variable, but I can't find where?
Thanks again for your help. The 'Action' tool is great, I just can use some help and practice how to start using it a bit more than the standard pre-programmed actions.