Full screen images with JS?

I am trying to make my Nivo Slider display full screen in different screen resolutions without impacting the quality of the images.

I have read up a lot about this and have tried the following CSS:

.nivoSlider img {
width: 100%;

This works in so far as it gets rid of the scrollbar… but it doesn’t make the image appear in the screen size, it just crops a corner of it.

As you can see on my page http://ellekorhaliller.co.uk/jQuery/index.html (a work in progress!), I have 2 images that are too small and one image that is a lot bigger than the screen resolution but I put them in to see how they could be scaled to fit.

Does anybody know a good way around this with Javascript?
And if this is not possible, please let me know so I can give up!

Many thanks in advance,