This link describe in more detail: https://support.google.com/a/answer/6371005?hl=en
Specially of note is:
To add, change, or remove a user's photo
- Sign in to your Google Admin console.
Sign in using your administrator account (does not end in @gmail.com).
- From the Admin console Home page, go to Users.
- In the Users list, find the user. If you need help, see Find a user account.
- Click the user’s name to open their account page.
- (Optional) To add a photo, click Profile photo.
- (Optional) To change a photo, choose Change photo.
- (Optional) To remove a photo, choose Remove photo.
- To continue adding or changing a photo, find the image file you want on your computer and select it. Changes save automatically.
Note: When you change a user's photo, the photo's visibility setting might change to Visible only to people I can chat with. Learn more about a photo's visibility setting.
Should you still find yourself stuck, go to https://admin.google.com and signin with your email and password. You can click the ? in the circle top right and get support direct from Google.
