How to Edit Existing User Accounts on Your MacBook
If you have administrator access, it’s a cinch to make changes to an existing user account from the Users & Groups pane in System Preferences on your MacBook. (Often, this is to assign a personalized account picture.) Follow these steps:
Click the account that you want to change in the Current User list.
Don’t forget, if the accounts in the list are disabled and you can’t select one, you must unlock the Users & Groups pane. Click the lock at the bottom left of the System Preferences window (and type your password, if prompted).
Edit the settings that you need to change.
Click the square Picture well to specify the thumbnail image that appears in the Login list next to the account name.
Apple provides a number of good images in the preview collection — just click a thumbnail to select it.
To add your own picture, click the Picture Well and click Edit Picture. You can drag a new image from the Finder or iPhoto window into the icon placeholder, or click Choose to select one from an Open File dialog; then click Set.
Alternatively, you can click the Take a Video Snapshot button to grab a picture from your MacBook’s FaceTime video camera. Most cool.
After you make the changes (and you select just the right image to capture the user’s personality — a more difficult task than you might think), press Command+Q to save them and close the System Preferences window.