They use the CSS Sprite technique to overlay a certain portion of this image 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.
As you can see, this question has been answered, but in the future you should consider using Firebug (http://getfirebug.com/). With firebug, you can inspect any element on a web page and see exactly how things are done. I've learned all sorts of tricks this way.
And if you're a Safari user, you can just right-click on something and choose "Inspect Element".