There isnt an actual css function that can give a background image to the selected size of the border, but if you have a look at your imported folder css file that you are trying to edit and find:
/* 2. POST BACKGROUNDS AND TEXT */
underneath it sets the colours of the table headers such as:
DIV.blockpost H2 {BACKGROUND-COLOR: #7EA34B}
so you could give that H2 a background colour by adding this:
DIV.blockpost H2 {background-image:url(../../img/yourimage.jpg)}
There the image im using is located in the img folder as you can see, but u can add it in anywhere
The only thing is that only adds a background image to the header H2, so maybe u can add a header at the bottom of the table too and do the same thing.
But im not sure if u are able to do the same with the left and right borders, u will have to make a template or something.