To change your image, click on your name (top right) and select 'account settings'.
Select 'change your picture'.
Press 'choose file' and navigate to a picture you like. Note it must be at least 100 by 100 pixels but no bigger than 4MB.
Press 'upload image'.
You will then get a chance to crop the picture to fit the space available.
Good luck!
I tried yesterday but failed as it kept going off to a "Nabble" page that didn't let me do anything. But today it worked fine and I even managed to download a picture without asking my children for help!