Is very easy
http://picasaweb.google.com/data/entry/api/user/<hereYourUserIdOrYourEmail>?alt=json
just has a little problem.
You only can get the picture of your Google+ profile, not directly from your gmail.
UPDATE:
This API is being deprecated and will be turned down in January 2019. Migrate to Google Photos Library API as soon as possible to avoid disruptions to your application.
More Info Here