Click on "Settings" up top of the page, then look for "Edit Avatar" on the left of the next page. Where do you have the picture you want as your avatar? Put a dot in "use custom avatar". Click on "browse" and browse to it. Once you've found it, click on it, then click "open". That should put the picture's full file path in the box along side the browse button. Click "save changes". That should give you the new avatar, if it meets the small size requirement, which I think is around 160x160 pixels.