[Answer]-Scale Django ImageField before saving to disk

I was close, but not quite there. This function works now fine and the image does not get saved to the disc at any point during the scaling.

IMAGE_MAX_SIZE = 800, 800
class Picture(models.Model):
    ...
    image = models.ImageField(upload_to='images/%Y/%m/%d/')

    # img is a InMemoryUploadedFile, received from a post upload
    def set_image(self, img):
        self.image = img
        self.__scale_image(self.image, IMAGE_MAX_SIZE)

    def __scale_image(self, image, size):
        image.file.seek(0) # just in case
        img = Image.open(StringIO(image.file.read()))
        img.thumbnail(size, Image.ANTIALIAS)
        imageString = StringIO()
        img.save(imageString, img.format)

        # for some reason content_type is e.g. 'images/jpeg' instead of 'image/jpeg'
        c_type = image.file.content_type.replace('images', 'image')
        imf = InMemoryUploadedFile(imageString, None, image.name, c_type, imageString.len, None)
        imf.seek(0)
        image.save(
                image.name,
                imf,
                save=False
            )