Go to the Notification option at the top of the site. Then click it and will choose Inbox one. In the new opened window scroll a little bit down your screen and find My Settings menu on left side. Then click Edit Avatar option there. In the next window you can change your one.
If you will click on the Forum Actions button at the top of the page and click Edit Profile, it should be self explanatory. If you have any problems just post back and I or some one else will give you a hand. Hope this helps.