The only way you can use your own image is if you have the old camera. I haven't tried it with the Kinect but with the old camera I took a picture of a drawing I made and my wife held up a photo of herself and snapped a pic. You can then use that as your
gamerpic. However as Trip said above, it's useless because the only people who can see it are you and your friends. Outside of that it's going to only show strangers your default Xbox live approved gamer pics from MS or the marketplace.