A. From the Account menu of the PokerStars lobby, select "Select/Change Image". Click on "Select Image", and pick an image for use from your hard drive. Obscene or inappropriate images are not permitted. Images are therefore subject to review and once submitted will not appear on your account for several days. For more information about images, and to find out how to take a picture from the Internet to use as your PokerStars image, visit our Images page.

Here is another link. This one points to their "images page".

http://www.pokerstars.com/poker/room/features/images/