class ImageQuerySet(models.QuerySet):

    def delete(self, *args, **kwargs):
        for obj in self:
            obj.img.delete()
        super(ImageQuerySet, self).delete(*args, **kwargs)

class Image(models.Model):
    objects = ImageQuerySet.as_manager()
    img = models.ImageField(upload_to=get_image_path)
    ...
    def delete(self, *args, **kwargs):
        self.img.delete()
        super(Image, self).delete(*args, **kwargs)