Fetching an image from google app engine ndb

Can someone verify/correct my method of accessing an image that is part of a ndb.Model class?

This is my MainPage handler:

class MainPage(webapp2.RequestHandler):

def get(self):
    guestbook_name = self.request.get('guestbook_name',
                                      DEFAULT_GUESTBOOK_NAME)
    greetings_query = Greeting.query(
        ancestor=guestbook_key(guestbook_name)).order(-Greeting.date)
    greetings = greetings_query.fetch(10)

    template_values = {
        'greetings': greetings,
        'guestbook_name': urllib.quote_plus(guestbook_name),
    }

    template = JINJA_ENVIRONMENT.get_template('index.html')
    self.response.write(template.render(template_values))

This is my index.html template code:

{% for greeting in greetings %}
  <div><img src="/img?key_id={{ greeting.key.id() }}" />
  </div>
{% endfor %}

This is my image handler:

class Image(webapp2.RequestHandler):
def get(self):
    greeting = ndb.get(self.request.get('key_id'))
    if greeting.avatar:
        self.response.headers['Content-Type'] = 'image/png'
        self.response.out.write(greeting.avatar)
        self.response.write('it works')
    else:
        self.error(404)
        self.response.out.write('No image')

The idea is that I store my image in a blob in my Greeting NDB Model. I then try to get that image by populating the main page with my MainPage handler which calls my Image handler. I pass the specific ndb entry key via my template and then retrieve the key from the template in the Image handler. I then attempt to display the blob attribute by getting the ndb entry using the key. Here's the error trace: