lets say you have a model called Book and on it a field called 'cover_pic', in that case, you can do the following to compress the image:
from PIL import Image
b = Book.objects.get(title='Into the wild')
image = Image.open(b.cover_pic.path)
image.save(b.image.path,quality=20,optimize=True)
hope it helps to anyone stumbling upon it.