How do I set an repeat image on each side of the window?

[talkncaasports]

Here is my website: http://www.olevetpossehideout.com/forums/index.php

Here is what I am trying to do.

On the left and right sides of the window, I have an image I want to use to will go over the top of the background image. However I do not want that image to take over the middle part. I want the image (grass background) to stay. I also want the background and the left and right sides to stay stationary.

1. Do I need to add sidebars?
2. If so, who do I do that? I am a novice when it comes to this stuff.

Thanks in advance.