It's rather simple, just follow these steps:
01) Select start
02) Select "Control Panel" OR "Settings"->"Control Panel"
(depending on the interface you have set up)
If you have the "expand" feature enabled for control
panel (default is off), select from the FlyOut menu, OR
select it, then double-click
03) "User Accounts"
04) Select the User who's picture you whish to change
05) Select the 4th option from the top in the main window
marked "Change my Picture".
The rest is quite easy to do...