first clickthen go to
then click 'Apperance and Themes' and then 'User Account Picture' (should be on the left)
Then a small pop-up should appear with a list of pictures
click on browse for more pictures and find the pic you want
When you've found it click open and then just exit the window
and you're done!
(to check it just go to start and it should appear next to your user name at the top)
then go to