Simple. Place your cursorover the "Edit" button on your "My Profile" page. A pulldown menu will appear that will include "Update Your Image". Click on this link. The next page will give you the option of adding your own image that you upload from your computer or selecting from a set of images provided. When you are done, select confirm choice and your image will be added.

You can change this image anytime after that by going to Edit -> Update your image and going through the same steps to change the image.