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.

set your background for the body the way you want and then set the background color for the various areas like header, sidebar, center and footer etc. the way you want them.