function showBack(div, no, img){
    div.style.backgroundImage = "url("+img+")";
    var f = document.getElementById("front_"+no);
    var b = document.getElementById("back_"+no);
    f.style.display = "none";
    b.style.display = "block";
}
function showFront(div, no, img){
    div.style.backgroundImage = "url("+img+")";
    var f = document.getElementById("front_"+no);
    var b = document.getElementById("back_"+no);
    f.style.display = "block";
    b.style.display = "none";
}