This is an old request, see e.g.
feature request: mass resize cover art pictures
I can only guess the reason why it has not yet been implemented:
MP3tag is a tagging program. It only deals with tags.
It does not edit the audio part - you need other programs for that.
It does not edit the picture - you need other tools for that.
Also: besides resizing there are numerous other functions that a user might want to have to deal with pictures right up to the full version of Photoshop. And that would be completely off course, I guess.