Go to your profile (there is a link to it on the bar at the top of the page) then from there you should see an option that says 'avatar' (it should be under the top bar mentioned above). Click on that and you should be able to change your avatar. You can upload a image, link an image off of the internet or choose a photo form your gallery. Please note that your avatar shouldn't contain anything obscene, or vulgar, and there is a limit on width, height, and size.
Short version: Profile>Avatar>Upload/Link/Gallery
Good luck!