A picture file may have metadata, just like an audio file.
When Mp3tag embeds a picture in an audio file, the metadata in the picture file is maintained.
It is possible that audio files may have embedded pictures that are missing metadata required to allow the pictures to be displayed by the player.
For example:
My car (Honda Accord) will not display embedded album art unless the embedded picture has these tags:
JFIF:ResolutionUnit: inches
JFIF:XResolution: 72
JFIF:YResolution: 72
The player does not display album art for audio files containing embedded pictures without these tags.