var max = 10;
var nrImages = 3;
function makeImages() {
this[0] = "/imgs/top_instituicao.jpg";
this[1] = "/imgs/top_instituicao2.jpg";
this[2] = "/imgs/top_instituicao3.jpg";

this.length = nrImages;
}

var vetImages = new makeImages();
var x = Math.round(Math.random()*max);
var y = max / nrImages;
for(var cont = 1;cont*y<= max;cont++) {
if (x <= (cont*y)) {
document.write("<img src="+vetImages[cont-1]+" border=0>");
break;
}
}