title: django serializer thumbnail of ImageField using sorl
body: <p>I have an existing serializer which includes an ImageField which can be very large image. I want to have a smaller thumbnail version of that image.
  I am trying to use sorl to create the thumbnails, since its able to upload it to AWS S3. How can I create this thumbnails in the serializer?</p>
  
  <p>in my current serializers.py:</p>
  
  <pre><code>class EmployeeSerializer(serializers.ModelSerializer):
      class Meta:
          model = Employee
          fields = ("id", 
                      "username", 
                      "first_name",
                      "last_name",
                      "face_image",)
  </code></pre>
  
  <p>my model looks like this, it uploads to AWS S3:</p>
  
  <pre><code>class Employee(models.Model):
      username = models.CharField(max_length=30, blank=False)
      first_name = models.CharField(max_length=30, blank=False)
      last_name = models.CharField(max_length=30, blank=False)   
      face_image = models.ImageField(upload_to=upload_to('employee/face_image/'), blank=True, null=True, storage=MediaStorage())
  </code></pre>
  
  <p>This is an attempt to have a new field in the serializer called "face_image_small", but it doesnt work</p>
  
  <pre><code>  from sorl.thumbnail import get_thumbnail
    class EmployeeSerializer(serializers.ModelSerializer):
          class Meta:
              model = Employee
              fields = ("id", 
                          "username", 
                          "first_name",
                          "last_name",
                          "face_image",
                          "face_image_small",)
  
          face_image_small = get_thumbnail(self.face_image, '200x200', crop='center', quality=99)
  </code></pre>