I've experiencing this issue (using v2.58)
with an album of mp3 which contain 1 embedded cover art .jpg 7 kb:
(there's no extra cover art as separate file)
in Mp3tag the cover art doesn't appear
neither in the bottom left corner, (when I select an mp3)
nor via rightclick mp3>Extended Tags.
In Options, by default the option "Don't display first image from file directory as cover art" is disabled.
If I enable it, then the cover art appears in the bottom left corner,
and in rightclick mp3>Extended Tags
but the Remove cover/Extract cover are greyed out.
PS. With winamp, by rightclick on an mp3>view file info
I am able to view the embedded file, and I can save or delete it.