how do I add a background image outside (behind) the center post/page area.

I don't know where to post this and I can't find a setting for it. (I posted it in a different area and didn't get any replies) I want to insert a background image that would affect everything but the center post/page area. I want my blog to match my HTML pages. So, I want http://www.ellenfinkelstein.com/pptblog/ to match http://www.ellenfinkelstein.com. In the latter, you'll see a blue gradient background image and that's what I want for my blog.