django - ImageField scaled-down on the fly

scaled.py

# models.py: overwritten model save method
def save(self, *args, **kwargs):
    if self.cover:
        img_string = StringIO()
        pil_image = Image.open(self.cover.file)
        if pil_image:
            res_image = pil_image.resize((1034, 1400))
            res_image.save(img_string, pil_image.format)
            # original image replaced by thumbnail
            self.cover.file = InMemoryUploadedFile(
                img_string,
                self.cover.file.field_name,
                self.cover.file.name,
                self.cover.file.content_type,
                img_string.len,
                self.cover.file.charset
            )

    super(Issue, self).save()