They use the CSS Sprite technique to overlay a certain portion of this image <a href="http://static.ak.fbcdn.net/images/ui/UIRoundedImage.png" rel="nofollow">http://static.ak.fbcdn.net/images/ui/UIRoundedImage.png</a> 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 (<a href="http://getfirebug.com/" rel="nofollow">http://getfirebug.com/</a>). 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.
<a href="http://static.ak.fbcdn.net/images/ui/UIRoundedImage.png" rel="nofollow">http://static.ak.fbcdn.net/images/ui/UIRoundedImage.png</a><p>They overlay that image on top.
Here's a good tutorial on the technique:
<a href="http://cssglobe.com/post/3714/css-sprites-rounded-corners/" rel="nofollow">http://cssglobe.com/post/3714/css-sprites-rounded-corners/</a>