Sign in and click on your profile page. From there, you will see a settings button form the drop-down menu. Select profile, select edit profile, and then make your changes.
It should take from immediately to a few seconds before your photo appears. Refresh the page or your server for the picture to show if you have a network issue.
You can cancel your profile. They will be sad to see you go, but if you feel like you don't need the service again, you can.