$(function(){

  var i = 0;
  var urls = [];

  $('#imagesMenu li span').each(function(){
    var url = $(this).attr('name');
    var image = new Image();
    image.src = url;
    urls.push( {url:url, element:$(this)} );
  });

  $('#mainImage')
    .css('cursor', 'pointer')
    .click(function() {

      i++;

      if(i >= urls.length ) 
        i = 0;

      $(this).attr('src', urls[i].url);
      $('#imagesMenu li span').css('font-weight', 'normal');
      urls[i].element.css('font-weight', 'bold');
    });

});