Hm, why just copy front cover PICTUREs from flac to mp3? The flac

PICTURE metadata block has the exact same metadata as the mp3 ID3v2
APIC frame; just copy them all.

flac.c:
    (flac_pictures): Return fields in the same order they're found in
    the FLAC format.  Convert the description to a unicode object
    rather than returning a UTF-8 encoded str object.

taglib.cc:
    (taglib_apic): Take a list of picture tuples and add them all as
    APIC frames.

    (add_apic_frame_to_mp3): Take the list of picture tuples and pass
    them straight to taglib_apic.

flac2mp3:
    (flac2mp3): Take a list of picture tuples instead of just one;
    pass them all to add_apic_frame_to_mp3.

    (find_pics): Rename from find_pic.  Save all PICTURE metadata
    blocks rather than just saving the first front cover picture.