Resizes, commits image to storage, and returns field's value just before saving.

def pre_save(self, model_instance, add):
        """
        Resizes, commits image to storage, and returns field's value just before saving.
        """
        file = getattr(model_instance, self.attname)
        if file and not file._committed:
            file.name = self._clean_file_name(model_instance, file.name)
            file.file = self._resize_image(model_instance, file)
            file.save(file.name, file, save=False)
        return file