img = new Array();
alt = new Array();

img[0] = "img/img.jpg";
img[1] = "img/img1.jpg";
img[2] = "img/img2.jpg";

alt[0] = "Exhibition image";
alt[1] = "Exhibition image";
alt[2] = "Exhibition image";

n = Math.floor(Math.random()*img.length);

document.write("<img src='"+img[n]+"' border='0' alt='"+alt[n]+"'>");