Go to MY ACCOUNT (at top right of page) click on it,
Go to SETTINGS TAB
click on browse to choose where your picture you want to use for your avatar is on your computer
or add in link if you are using pic from file hosting site.
or pic one from the ET avatar collection
once thats done scroll down to bottom of the page and click save changes.
Remember that avatars can be no larger than 2 MB and pixel size of 100x100p