How to: Put Image Behind Page Body Text

Hi,

I'm wondering how to put an image behind my main page body in a WPMUDEV theme. I'm referring not to a background image, but something that would go where the main body color usually appears.

An example is on the basic Rustic theme http://wordpress.org/extend/themes/rustic

My client likes the paper that appears behind the blog entries etc. I need to find a way to recreate it.

Can anyone help?

Thanks
Kimberly