The methods you are using to make this site are very outdated now, tables are very hard to work with when you are trying to gain control over the style.
One way you do it with CSS is to position 2 elements at the corners of the container element with absolute positioning and background images.
However I tend to nest elements wherever I need more than one background image though.
This would create 3 box level elements in exactly the same spot with the same dimensions - All you need to do is apply the background images with a transparent background-color so you can see the image underneath.
<div id="header"> -- This can hold the background
<div class="inner"> -- This can hold one of the corner images
<div class="inner"> --This element can contain the other corner image
Content goes here