Size Parameter doesn't work on Google CDN for images

I am using webApp2 and Google App Engine for a simple app and images and blobstore api to serve uploaded images using Google CDN. I am using the following code to generate the Serving Url for images -

from google.appengine.api import images
from google.appengine.ext import blobstore

def _get_urls_for(self, file_name):
    user = users.get_current_user()
    if user is None:
        return

    bucket_name = app_identity.get_default_gcs_bucket_name()
    path = os.path.join('/', bucket_name, user.user_id(), file_name)
    real_path = '/gs' + path
    key = blobstore.create_gs_key(real_path)
    url = images.get_serving_url(key, size=0)
    thumbnail_url = images.get_serving_url(key, size=150, crop=True)
    return url, thumbnail_url

It gives me the url as following (as example) -

http://localhost:8080/_ah/img/encoded_gs_file:YXBwX2RlZmF1bHRfYnVja2V0LzE4NTgwNDc2NDIyMDEzOTEyNDExOC9pbWc4NzYuanBn=s32

and

http://localhost:8080/_ah/img/encoded_gs_file:YXBwX2RlZmF1bHRfYnVja2V0LzE4NTgwNDc2NDIyMDEzOTEyNDExOC9pbWc4NzYuanBn=s150-c

Both the urls defer by just the last size parameter (=s150-c) which should crop the image in the thumbnail_url to 150 px as it's largest dimension. I can confirm that by referring to the official documentation here. However the parameter doesn't affect the size of image at all. I am still getting full size image, even after adding the parameter. I have already tried to remove -c and tried with various sizes.
I have tried few arguments from this list as well, which unfortunately doesn't affect the image at all.

Any idea what could be the reason of this behaviour?

After tracing the log, I found out that it happened due to PIL not being installed locally. If the transformation needs to be done on local server, then PIL needs to be installed. I chose to install Pillow by using pip install Pillow -t lib/ and then added the following to app.yaml

libraries:
- name: PIL
  version: "1.1.7"

Then added an appengine_config.py file in root of my project where lib folder resides and added the following code to it.

# appengine_config.py
from google.appengine.ext import vendor

# Add any libraries install in the "lib" folder.
vendor.add('lib')

And it started to work magically.