At the top of any page on the site go to the link "Profile"And then to the link " Avatar". From there you have some options.

Upload an Avatar from your computer : Click "Browse" Then go through your files and select the picture that you want." Then click "Save."

Upload an Avatar from an URL : Copy the URL for the picture/avatar/icon that you want and paste it in the text box. Then click "Save."

Link to an off-site Avatar : Same instructions as Upload an Avatar from an URL