function changeImage() {
          newImage = "url(img/back_pool.jpg)";
          document.getElementById('back_pool').style.backgroundImage = newImage;
}
function changeImage2() {
          newImage2 = "url(img/pool2.jpg)";
          document.getElementById('back_pool').style.backgroundImage = newImage2;
}
function changeImage3() {
          newImage3 = "url(img/pool3.jpg)";
          document.getElementById('back_pool').style.backgroundImage = newImage3;
}
function changeImage4() {
          newImage3 = "url(img/pool4.jpg)";
          document.getElementById('back_pool').style.backgroundImage = newImage3;
}