Border blog body

Hey all,

I want keep the ‘main body’ of my blog white, but have a background image as the background.

At the moment, if i put a background image on to the blog, the entire page gets imaged, so it’s hard to see the actual blog.

Here’s my blog: https://schristian.eu.

So i want the main, middle; to be white, but have a background image.
Here’s an illustration of what i’d like: https://img38.imageshack.us/img38/3889/blogpreview.jpg.

Can anybody guide me? ??

Thanks!