1. Once you are logged in to your online email session in Google, click on the gear icon in the upper right-hand portion of your screen. 
    Screenshot showing the small gear icon in the top, right corner of the email window.
  2. Click on the 'Settings' link.
  3. On the 'General' tab, scroll down and find the 'My Pictures' section.
  4. Click on 'Change Picture'.
  5. Click on 'Choose File' to select a picture from your computer. (If your computer is equipped with a camera, you may have the option of taking a 'selfie' from your computer.)
    Screenshot of the 'upload image' window.
  6. Once the image is selected, a box will be positioned directly over the picture that will allow you to resize the image it by dragging on the corners. 
  7. Then, all you have to do is apply the changes. Now you can decide whether ALL Gmail users are going to see your photo or only those who will chat with you.

https://support.google.com/mail/answer/35529?hl=en