How are cover pictures tagged to an .mp3?

I downloaded the software a few days ago and very quickly found out how to use the parts I need. The numbering wizard is so helpful. It didn't seem to be available altering the tags in Windows 10.

One question: How are the cover pictures tagged to the file? I've found out how to do it, but I expected a tag giving the picture's file name. I could not find one. Also, does the picture file need to be present? I don't think so, because one folder will only allow one cover.jpg. However, the correct photo will always show with different .mp3 files in the same folder. Come to think of it, the photo shows when no picture file is present.

There are basically two ways:

  1. Embed the picture into the mp3 and you don't need any additional external file
  2. Don't embed the picture and ensure, that every player you use will read the correct picture for your played song.

Obviously, I would suggest to choose way 1)

Maybe some more technical background information:
The embedded picture will be saved into the APIC tag inside the mp3 metdata.
For details please see 4.15 at id3v2.3.0 - ID3.org

Just to append option 2: that is not really tagging. That is what a player may make of it. Showing a picture is a player feature and not so much a problem of how a picture is added.
If it is a decent player, then it will show the embedded picture for each file.