Go to the upper right hand corner where it has notifications, My profile, settings, log out.
Select settings.
Go to left hand side of page and scroll down past My Messages and My subscriptions to My settings.
Under My settings select Edit Profile
From there Option 1 you will have to enter your URL to an image you have selected as your Profile.
Option 2 you click browse and select a picture you have saved in your picture file and click save changes.
Sometimes you have to adjust the size and there are programs that will do that. I don't have a link handy at the moment to such a site that will allow you to adjust the size of your pic.
Hope that helps.