To access your settings page you must be logged in. Then, on the upper right corner of the site you'll see your profile picture, your name, a link to your settings page and a link to log out.
Click on the "settings" link to open your settings page. Then choose the option picture on the top of the page.
On this page you can see your current picture if you have one, and you can upload a new picture to replace it. The uploaded image will be automatically resized and cropped to the correct size.
Sometimes it can take a few minutes untill you see your new picture. Please be patient and refresh the page.