set a default with css
add:
//stops the image from being larger than 100% width
.nivoSlider img {max-width: 100%!important}
or
//stops the image from being larger than 100% width
body .nivoSlider img {max-width: 100%}
or perhaps this, to override it in browsers that don't support max-width
if needed:
//sets width to 100% by default
.nivoSlider img {width: 100%}
to your css, to override:
.nivoSlider img {max-width: none}
in nivo-slider.css