It's a background image set in the document body.

Here's the CSS code for it:

body
{
background-image: url(image.gif);
background-position: top right;
background-repeat: no-repeat;
}

These are the 'bare' essentials of the CSS centering hack: the width and the margin. If you don't specify a width, the margin trick won't work. If you don't specify the left and right margins as auto...