title: Retrieving a user's public google/gmail picture
body: <p>I'm writing an email-based application using the gmail-api, and i need to retrieve user public images for email address originating from google.
  I've seen other applications do that, but i can't seem a way to reliably get this data.</p>
  
  <p>some more information:</p>
  
  <ul>
  <li>i'm using oauth2 to sign the user in - and i have no problem requesting any type of permissions there.</li>
  <li>i've tried retrieving the image using the google+ api - but it requires a userID for that - and i don't know how to get the userId for the email address (gmail-api doesn't give it, afaik).</li>
  <li>i've tried using the contacts API - but it only gives images for the user's contacts, while the email clients can display images from other users as well.</li>
  </ul>
  
  <p>Thanks!</p>
  
  <p>Edit:</p>
  
  <p>I know i can receive the required images from google plus, if i have the other user's userid. however, i can't find a way to get the userid based on an email address.</p>