add_to_before_end_head.html

<style type="text/css">
<!-- 
#box p { height:100% }
#box p, #box p img { margin:0; padding:0 }
-->
</style>

add_to_protaculous.js

$('box').select('img').invoke('writeAttribute','height',null).invoke('writeAttribute','width',null);
var fixPhoto = function(){
  var img = $('box').down('img');
  var screen = document.viewport.getDimensions();
  if(img.getWidth() > screen['width']){
    img.setStyle({width: '100%', height: null});
  }else{
    img.setStyle({height: '100%', width: null});
  }
};
fixPhoto();
Event.observe(window,'resize', fixPhoto);