How to add a ManyToManyField to a field?

Question

Django

I created a ManyToManyField field

class Picture(models.Model):
  link = models.CharField('picture_link', max_length=200)
  def __str__(self):
    return self.link

class MyModel(models.Model):
     picture = models.ManyToManyField(Picture, blank=True)

And I'm trying to add a model to the field

for pic in arr:
  _picture = pic.text
  try :
    picture = Picture.objects.get(link = _picture)
  except:
    picture = Picture(link = _picture)
    picture.save()
  mymodel.picture.add(picture)

But I am getting an error

Cannot add "<Picture: https://nndd.com/wa-data/public/shop/products/29/13/1329/images/538/538.602x0.jpg>": instance is on database "None", value is on database "default"