Insert a Logo in the Background

They defined the background image in the CSS for a specific div block that is placed at the top of their content div block. I have no idea what theme they used but I think you could get the same effect by creating an empty block that is put at the top of all nodes, and then use the same CSS they use to put it in the background, i.e.,

#bg-seal {
position: absolute;
background: url(images/seal-bg.png) no-repeat;
left: 80%;
top: 20px;
width: 306px;
height: 500px;
text-indent: -9999px;
z-index: 0;
}