var photo_length = photo_info.length/2;

function get_random()
{
  var ranNum= Math.floor(Math.random()*photo_length);
  return ranNum;
}

var photo_num = get_random()*2;

document.writeln("<div><img src='" + photo_info[photo_num] + "' width='340' /><div class='caption'>");
document.writeln(photo_info[photo_num+1] + "</div></div>");