function over(id) {
  switch (id) {
    case 1:
      elem = document.getElementById('link1');
      elem.src = "images/layout_03_over.png";
      break;
    case 2:
      elem = document.getElementById('link2');
      elem.src = "images/layout_05_over.png";
      break;
    case 3:
      elem = document.getElementById('link3');
      elem.src = "images/layout_06_over.png";
      break;
    case 4:
      elem = document.getElementById('link4');
      elem.src = "images/layout_08_over.png";
      break;
  }
}
function out(id) {
  switch (id) {
    case 1:
      elem = document.getElementById('link1');
      elem.src = "images/layout_03.png";
      break;
    case 2:
      elem = document.getElementById('link2');
      elem.src = "images/layout_05.png";
      break;
    case 3:
      elem = document.getElementById('link3');
      elem.src = "images/layout_06.png";
      break;
    case 4:
      elem = document.getElementById('link4');
      elem.src = "images/layout_08.png";
      break;
  }
}