// preload images that will be displayed in tooltip
// put images to preload in this array
if (document.images) {
    var preload_list = new Array();
    for (var preload_counter=0; preload_counter<image_list.length; preload_counter++) {
    preload_list[preload_counter] = new Image();
    preload_list[preload_counter].src = image_list[preload_counter];
  }
}