Windows XP introduced support for User Account Pictures (there are 24 of them). Here's how to change your User Account Picture in Windows Vista.
Step 1: Step 1: Control Panel
Click on the Start button, then click on Control Panel, and you will see the User Account and Family Safety category.
Step 2: Step 2: User Accounts
After clicking on User Accounts and Family Safety, click on User Accounts.
Step 3: Step 3: Change Your Picture
After clicking on User Accounts, click on "Change your picture", click on one of the pictures (or click the Browse button and find your picture, then click Open), click OK, and click the Close button on the title bar.
Tip: For your picture, we recommend you crop the image to 128x128.