I'd recommend mutagen, it's a pure python library with no other dependencies and it supports a lot of different audio metadata formats/tags (MP3, FLAC, M4A, Monkey's Audio, Musepack, and more). To extract artwork from an ID3 v2.4 MP3 saved with iTunes:
from mutagen import File
file = File('some.mp3') # mutagen can automatically detect format and type of tags
artwork = file.tags['APIC:'].data # access APIC frame and grab the image
with open('image.jpg', 'wb') as img:
img.write(artwork) # write artwork to new image