Want to use Featured Image as Background in BODY BOX only

As the title suggests, I want to use Featured Image as Background in BODY BOX only, on the main page.

I would assume that you can call the featured image somehow, but it is a little beyond me. If you can help, it would be best for all (well at least me) if you explained it as if you were tlking to a 5 year old, because I tend to miss the little things.

Blog is here. http://wotn.tuskland.info

As you can see, on my first post there is a featured image. What I would like to do is remove the featured image that is currently being displayed, then use the same image as the background image in the body box in the center area.

Obviously, the featured image would change by post, so just calling the url of the image doesn't really fit my needs.