Click on "My Profile" in the top (menu) toolbar then look right under your existing avatar and it will say "Replace Avatar" just click on that to change the picture.
A new box will open with 2 tabs (1-default) asking you to either browse your computer for an existing photo or (2) Image URL
Once you have done one of the 2 steps above to add or upload a new photo, and click "submit", it will change your avatar.