They use the CSS Sprite technique to overlay a certain portion of this image http://static.ak.fbcdn.net/images/ui/UIRoundedImage.png http://static.ak.fbcdn.net/

They use the CSS Sprite technique to overlay a certain portion of this image http://static.ak.fbcdn.net/images/ui/UIRoundedImage.png http://static.ak.fbcdn.net/images/ui/UIRoundedImage.png on top of your picture. It's hard to see in the browser but above the blue rounded corner mask there is a white corner mask.