how do you change your xbox live gamer picture to a custom image

Hey, i was wondering how to change the gamer picture to a custom image. Ive seen people with pictures that arnt actually set by microsoft if you know what i mean, such as 'the joker' and chuck norris.

I'm guessing most people used their Vision Camera and took a shot of a picture they liked. As far as I know the only custom images you can use are from the Vision Camera.