How to add a social network profile image as an overlay?

In order to add an overlay of a social network profile images (e.g., from Facebook, Twitter, Gravatar.), you should set the name of the network before the profile's user ID, for example:

Facebook user ID (facebook):
http://res.cloudinary.com/demo/image/upload/w_200/l_facebook:65646572251,w_50/sample.jpg

Twitter user ID (twitter):
https://res.cloudinary.com/demo/image/upload/w_200/l_twitter:1330457336,w_50/sample.jpg

Twitter also supports profile names (twitter_name):

http://res.cloudinary.com/demo/image/upload/w_200/l_twitter_name:BillClinton,w_50/sample.jpg

For more information, please see the Social Media Profile Pictures documentation