BgStreacher plugin for WordPress implements the great bgStretcher Jquery script to your wordpress blog.
It allows you to add a large image (or a set of images) to the background of your web page and will proportionally resize the image(s) to fill the entire window area. The plug-in will work as a slideshow if multiple images mode is used (the speed and duration for the slideshow is configurable)..
For example, google uses such a hack for their background image feature
The plugin supports all the bgStreacher features, like slideshow.