it's not that difficult. In the main menu bar above your posting, choose "Go" --> "My Space" --> "Profile". On the following page choose "View/Edit Complete Profile". Under "Public Profile Fields" look for the item "Avatar / Picture URL:". Choose "Edit". You can now choose an avatar from the newly opened page (or one of the following pages if you select "next") or you can use a custom avatar which must be uploaded onto a web space before (select "Custom Avatar" for this). When you have made your selection, choose "Select This One" - and that's it!
Hope this helps.