go to 'settings'
on the left hand side, in the 'My settings' box, there is the option 'edit avatar'.
probably eaiest to use option 2 - save the picture you want to use somewhere on your pc (my documents or the pictures file usually) - give it an easy to remember name so you can find it easily.
click on 'browse', find the picture you want to use on your pc and double click on it.
then click on 'save changes'.