$(document).ready(function(){
  var conf = {
    className : 'ro',
    postfix : '_on'
  };
  $('img.'+conf.className).hover(function(){
    this.originalSrc = this.src;
    this.src = this.originalSrc.replace(/(\.gif|\.jpg|\.png)/,
		conf.postfix+"$1");
  },function(){
    this.src = this.originalSrc;
  });
});