jquery supersized plugin - allow image to scroll with site

I'm developing a site and the client has requested we use the supersized plugin to display a gallery of images - which is fine, HOWEVER the have requested that the background image scrolls with the site rather than being fixed to the browser window. Something like this - http://www.surfcampinportugal.com/ .

I've tried amending my CSS (changing position: fixed to absolute) but nothing seems to achieve this effect.

Site is on my local server at the moment but I'm using supersized 3.2 with a fade transition - http://buildinternet.com/project/supersized/slideshow/3.2/demo.html

Any pointers/ideas would be VERY welcome.

Thanks