You can use CSS3 background-size to make your background-images stretch to cover the area of the box.
If you then create a containing box positioned absolutely over the whole page, you can put your slides in there.
At that point it is just a matter of fading them in and out, something which can be easily done via a jQuery plugin like Cycle.
Here’s an example: http://jsfiddle.net/ZnAqE/1/
As you can see all you need to do to add slides is create a new child div for #slideshow. In WordPress this can be done by looping over your gallery images.
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