Do you want it to display as a background for the whole page?
If so, go to the style you want to edit and in 'General CSS Properties' expand 'body' and add:
background-image: url(http://topkayaker.net/styles/images/06BLUE.jpg);
background-repeat: repeat;
repeat - The background image will be repeated vertically and horizontally
repeat-x - The background image will be repeated horizontally
repeat-y - The background image will be repeated vertically
no-repeat - The background-image will be displayed only once