Update your user picture using Photo Booth on Mac
Use Photo Booth to update the picture on your card in Contacts and the picture that appears next to your name in the login window on your Mac.
In the Photo Booth app on your Mac, take a photo.
Select the thumbnail of the photo you want to use.
Click the Share button , choose Change Profile Picture, click Contact Card & User Picture, then click Set.
You can change your user picture in Users & Groups preferences. See Change a user's picture.