function changeImage() {
          newImage = "url(img/back_spa.jpg)";
          document.getElementById('back_spa').style.backgroundImage = newImage;
}
function changeImage2() {
          newImage2 = "url(img/spa2.jpg)";
          document.getElementById('back_spa').style.backgroundImage = newImage2;
}
function changeImage3() {
          newImage3 = "url(img/spa3.jpg)";
          document.getElementById('back_spa').style.backgroundImage = newImage3;
}