Using Vista for over a year, and I wasn't aware of this feature...
http://www.lockergnome.com/windows/2008/11/26/change-your-account-picture-in-vista/When you create a user account in Vista, it is assigned a log in picture that is displayed on the login screen. If you do not want to be a flower, you can easily change the picture to either one of the stock photos included with Vista or your own photo.
To change your user account picture:
1. Click the Start menu.
2. Click the picture icon currently assigned to your account. The User Accounts window will open.
3. Click the Change your picture option.
4. To use one of the stick photos, click the photo you want to use and click Change Picture.
5. To use one of your own pictures, click Browse for more pictures.
6. Locate and select the picture you want to use and click Open.
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