var rand1 = 0;
var useRand = 0;
images = new Array;
images[1] = new Image();
images[1].src = "images/fondo_3.jpg";
images[2] = new Image();
images[2].src = "images/fondo_3b.jpg";
images[3] = new Image();
images[3].src = "images/fondo_3c.jpg";
function swapPic() {
var imgnum = images.length - 1;
do {var randnum = Math.random();
rand1 = Math.round((imgnum - 1) * randnum) + 1;} 
while (rand1 == useRand);
useRand = rand1;
document.randimg.src = images[useRand].src;}
window.onload=swapPic;