Gday...

You can click on either your login name within the box "Logged in as" - or on the dark blue banner, click on User Details.

That will open up a list of items you can access - on the left side of the page.

Click on 'Avatar' which will open up an area where you can upload a photo from your computer to be your Avatar.

Be aware that this needs to be a low resolution pic - a Kb size not a Mb size - the smaller the better.

When the pic is uploaded it will appear in the 'box' shown. 

Cheers - John