class ImageModel(models.Model):
    image = ImageField(...)

    def delete(self, *args, **kwargs):
        # You have to prepare what you need before delete the model
        storage, path = self.image.storage, self.image.path
        # Delete the model before the file
        super(ImageModel, self).delete(*args, **kwargs)
        # Delete the file after the model
        storage.delete(path)