I was looking at newgrounds.com, and I saw that the backgrounds of their images exist out of only 1 image. - http://img.ngfiles.com/box.gif
Yet, they have rounded corners on both sides (even though the backgound is way bigger) and and their boxes expand.

I've looked at the code, but I cant really see it.
Can someone please explain me?

The bottom of their boxes also perfectly closes, with same image.

Thanks alot!