To access your settings page you must be logged in. On the upper right corner of the site you'll see your profile picture and your name. Mouse over your name to see more options:
Click on the "account settings" link to open your settings page. Then choose the Picture link in your settings menu:
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 cropped and resized to the proper size.
Sometimes it can take a few minutes untill you see your new picture. Please be patient and refresh the page.