var Fondo= new Class({
			options:{
				contenedor: {},
				imgs:[]
			},
			initialize: function(options){
				this.setOptions(options);
				this.muestra();
			},
			muestra: function(){
				longImgs= this.options.imgs.length;
				rand= Math.random();
				rand2= rand*longImgs;
				index= Math.floor(rand2);
				imgFondo= imgs[index];
				this.options.contenedor.setStyle('background-image','url('+imgFondo+')');
			}
});
Fondo.implement( new Options);