Set ImageField default value to random image out of a list of images in Django

I have some images inside the static directory and want to create a model that has a ImageField. I want to set the default field to any one of those images. I have tried using this -

def randomImage():
    return ImageFile('media/blog/image/' + str(random.randrange(1, 15, 1)) + '.jpg')

# ----------------------- Model for each post in the blog-------------------
class Post(models.Model):
    heading = models.CharField(max_length=150)
    author = models.ForeignKey(User)
    postBody = models.TextField()
    postDate = models.DateTimeField('posting date')
    postImage = models.ImageField(upload_to='media/blog/image/'+str(int(time.time())), default=randomImage)

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