title: in Model.save(), how can i check if a ImageField has changed?
body: <p>in Model.save(), how can i check if a ImageField has changed?</p>
  
  <pre><code>class Product(models.Model):
      img = models.ImageField()
      def save(self, *args, **kwargs):
          if (img is changed):
              #resize new img 
          else:
              #do nothing
  </code></pre>