Go to your username at the top right of the screen. Either hovering over it or clicking on it will give you this menu screen.
Choose "Avatar" in the left column, second from the bottom.
Select the "Choose File" button. From there you'll have to select the picture you would like to use. Mine may be a little different from your screen, so I didn't want to snap a picture and make things confusing.
Once you've selected your picture, click on the "Save Changes" button. Done!