if (document.images) {

   // Menu
   btn_home_on            = new Image();
   btn_home_on.src        = "images/button_menu_on.jpg";
   btn_progetto_on            = new Image();
   btn_progetto_on.src        = "images/button_menu_on.jpg";
   btn_servicies_on        = new Image();
   btn_servicies_on.src    = "images/button_menu_on.jpg";
   btn_solutions_on       = new Image();
   btn_solutions_on.src   = "images/button_menu_on.jpg";
   btn_housing_on        = new Image();
   btn_housing_on.src    = "images/button_menu_on.jpg";
   btn_data_on            = new Image();
   btn_data_on.src        = "images/button_menu_on.jpg";

   // Menu
   btn_home_off            = new Image();
   btn_home_off.src        = "images/button_menu_off.jpg";
   btn_progetto_off            = new Image();
   btn_progetto_off.src        = "images/button_menu_off.jpg";
   btn_servicies_off        = new Image();
   btn_servicies_off.src    = "images/button_menu_off.jpg";
   btn_solutions_off       = new Image();
   btn_solutions_off.src   = "images/button_menu_off.jpg";
   btn_housing_off        = new Image();
   btn_housing_off.src    = "images/button_menu_off.jpg";
   btn_data_off            = new Image();
   btn_data_off.src        = "images/button_menu_off.jpg";
}

function tnImgOn(imgName) {
   if (document.images) {
      document[imgName].src = eval(imgName + "_on.src");
   }
}

function tnImgOff(imgName) {
   if (document.images) {
      document[imgName].src = eval(imgName + "_off.src");
   }
}