To add an avatar image and change your display name, click your name at the top of the screen when you are logged in to your account.

To change your avatar once you are on the Settings page, click the edit link (pencil icon) next to your current avatar and upload your desired image.  

Once uploaded, adjust the circle to the location on the image you wish to use as your avatar and hit the "Submit" button. Note you can change the size of the circle by dragging the edge in or out to match the area you would like to use.