It's very easy. The only limitation is that the picture have to be 90x90 pixels and not bigger than 9kb.
At the main bar there is a link to the "User Control Panel":
Click there, and select "Profile" from the menu:
Then a second menu will show up. Go to "Edit Avatar"
and you will see the option to upload your image:
Click on "Choose File" and select your file in the pop-up window.
And the click on Submit...
And that's it! Have fun!
