var pixs = new Array();
pixs[0] = "images/image2.gif"
pixs[1] = "images/image3.gif"
pixs[2] = "images/image4.gif"

function randomNumber(n){
  number = Math.floor(Math.random() * (n + 1));
  return number;
}

document.write("<img src='" + pixs[randomNumber(pixs.length - 1)] + "'>");