Resizing Slideshow

Hi there,

I have the plugin working on a slideshow of images here:

http://samsweet.info/

I’ve uploaded all the images at the desired full size; all fall within 600 X 500 px. If I don’t specify the height/width in the shortcode, the images are stretched and blown out to fill the entire content ares. But when I specify the height and width in the shortcode, the images are not responsive and do not “shrink down” when the window is resized. Can someone guide me on how to get these images to (a) show up as they are at full size, centered and (b) resize properly in a smaller window/mobile device?
Any help appreciated!

http://wordpress.org/plugins/nextgen-nivoslider/