This request is an old one:
Re-size embedded artwork
Feature Suggestion
and many more
As far as I understood mp3tag it is for tagging. mp3tag does not modify the encoding, nor does it allow to edit the audiodata (or video data for mp4) nor is it a graphic tool.
As there are many more tasks circling around manipulation of graphics (see e.g. Photoshop) it would be rather hard to get the ghost back into the bottle.
I do not write this to discourage you or to criticize your suggestion - it is just my personal opinion.