I found this in a few different places around the 'net - hopefully it's helpful:
Change the Picture on the Welcome Screen
If your computer is a member of a workgroup or is a stand-alone computer, you can replace the picture on the Welcome screen with another picture. First, store a head shot that you've scanned or taken with a digital camera in the My Pictures folder. The picture will be automatically resized to fit the picture box, so don't worry too much about getting an exact size, and then follow these steps:
- Click Start, click Control Panel, and then click User Accounts.
- Click your account name, and then click Change My Picture.
- Click the picture you like, and then click Change Picture.
- To find the user's picture, click Browse for more pictures, click the picture you want to use, and then click Open.
Now, when you open the Welcome screen, you will see the new picture.