Slideshow background images in Background
There are few jquery library which are not using images directly. They are using images as css background.

I found one link for you - http://demo.marcofolio.net/bgimg_slideshow/

The following tutorial might be useful for you- 
http://www.techerator.com/2010/11/how-to-make-a-css-background-slideshow-with-jquery/

This tutorial explains how to use css background images for slideshow.