You need to create one image and repeat it in the background:
A. Theme Options -> Body, Text & Links -> Body Style
background: #cccccc url(/wp-content/themes/atahualpa/images/bg.gif) top center repeat-y;
I've attached the background image of my theme Willy the Cat below. It is made for a 740px wide layout though. Either look at the image and create a new one, or open that image in an image editor, copy one of the two shadows and move it farther away from the other shadow until you got the right width for your layout. Distance between [right end of the left shadow] and [left end of the right shadow] = width of your layout.