function VecImagenes(){
n=0;

this[n++]="url('" + ruta + "1.jpg')";
this[n++]="url('" + ruta + "2.jpg')";
this[n++]="url('" + ruta + "3.jpg')";
this[n++]="url('" + ruta + "4.jpg')";
this[n++]="url('" + ruta + "5.jpg')";
this[n++]="url('" + ruta + "6.jpg')";
this[n++]="url('" + ruta + "7.jpg')";
this[n++]="url('" + ruta + "8.jpg')";
this[n++]="url('" + ruta + "9.jpg')";
this[n++]="url('" + ruta + "10.jpg')";
this[n++]="url('" + ruta + "11.jpg')";
this[n++]="url('" + ruta + "12.jpg')";
this[n++]="url('" + ruta + "13.jpg')";
this[n++]="url('" + ruta + "14.jpg')";
this[n++]="url('" + ruta + "15.jpg')";
this[n++]="url('" + ruta + "16.jpg')";
this[n++]="url('" + ruta + "17.jpg')";
this[n++]="url('" + ruta + "18.jpg')";

this.N=n;
}
function cambia_imagen(){
var Imagenes=new VecImagenes();
src= Imagenes[ Math.floor(Math.random() * Imagenes.N) ] ;
document.getElementById("imagformul").style.backgroundImage = src;
}