Google Chrome OS
If you are talking about changing a user account picture on Chrome OS (Google's operating system), a simple search will find that information for you on Google's Help site. Below is from the previous link.
Change your account picture
- If you haven’t already, sign in to your Chrome device.
- Click the status area in the lower-right corner, where your account picture
- Select Settings and find the “Users” section.
- Click the
picture for your account. In the “Change picture” dialog that appears,
you can reset your account picture:
- Choose a file: Click the icon to
use an image saved on your Chrome device or a connected external
- Take a photo: Click the icon to take a new snapshot using a
webcam. Strike your favorite pose, then click the button. To flip
your photo horizontally, place your cursor over the photo and click on
the arrow icon in the lower-right. If you're dissatisfied with the
photo, click the button to retake it, as many times as you like!
your Google Profile photo: Use the same photo associated with the
public profile for your Google Account. Learn more about Google
- Use an icon: Do you think puppies are cute? Are you an avid
cyclist? Choose from a variety of fun icons.
Google Chrome Browser
To change an account picture on Google Chrome (the browser), follow the directions below:
- login as teh use you want to change the picture for.
- Left-click on the icon of the use in the top-left corner.
- Hover your mouse over the account you would like to change the picture for and click "Edit".
- From the "Edit user" menu, you can change the name, add/remove a desktop shortcut, and change the picture associated with the user account.
At this time is it not possible to use your own custom picture in Google Chrome, but it is possible to use your own custom picture in Chromium as per this link. This Superuser question asked the same question, but at the time, the answer was "no". Now the answer is "yes", but only for Chromium.