You can add your profile picture during the sign up process. Just click on the avatar and it will let you select a picture from your computer or mobile phone.

You can also add or change your profile picture after you've signed up. To do so, login to your account, and click on the avatar at the top right corner. It will open your profile page (see image below). Just click on your avatar and it will give you the option to select a picture from your computer or mobile phone.