Your profile picture, also known as an avatar, can be updated from your "Profile" page.  You can access your profile from the "Profile" link in the upper right hand side of site in the Personal Navigation Bar.  When you are in your "Profile" page, simply click the "Update My Picture" link below your current picture and follow the instructions to upload a picture directly from your computer.

(Note:   You need to be signed into the site to see the "Profile" link.  If you do not see the word "Profile" in the upper right-hand corner of the site, then you likely need to sign in.)