How to change the picture on a user account of your Mac if you'd like to give it a custom or more personalized feel.
2. Change User Picture
You can change the picture that appears next to a user name in the login window.
Click the Apple menu (1), then click System Preferences (2).
Click Users & Groups.
Click the User account (1) you would like to make changes to. If settings are locked, click the Lock icon (2) to unlock the settings, then click the User account picture (3) to begin making changes.
A window will appear that will allow you to browse for or capture a new photo for use as your User picture. Once you have selected the User picture you desire, click Done.