To access system preferences:
Apple menu > System Preferences
To change user picture:
Steps:
1. Apple menu > System Preferences > click Users & Groups.
2. You can change your own picture without unlocking Users & Groups preferences.
But if you need to make admin user changes:
Click the lock icon to unlock it, then enter an administrator name and password.
3. Select the user account on the left, then click the picture on the right.
Note: instead of clicking the picture, you can drag an image file from the Finder onto the picture.
For more informations, see here.
