From Henning:
1. In the template-parameters turn the page-suffix-feature to "on"
2. choose different "page-suffixes" for different menu-items like "bg-page-1", "bg-page-2" etc.
(you find this in the menu-items on the right in "parameters-system"
3. create a folder called "backgrounds" in the images-folder of your template and put your images into it.
4. add this to your css (I would use the style-css-files)
body.bg-page-1 {background: url(../images/backgrounds/img-sky.jpg) no-repeat 50% 0;}
body.bg-page-2 {background: url(../images/backgrounds/img-fire.jpg) no-repeat 50% 0;}
5. clear your cache ...