ヘッダー画像をランダムに・月ごとに替える

（デザイン／カスタマイズ／ヘッダ／タイトル下）（「爺の時悠帖」）

<script>
today  = new Date();
MM = today.getMonth();
bg_img = new Array();
  bg_img[0] = "url(https://….jpg)"; /* 月ごと画像設定なしのときのデフォルト画像 */
switch (MM) {
case 0:  /* 1月の画像 */
  bg_img[0] = "url(https://….jpg)";
  bg_img[1] = "url(https://….jpg)";
  bg_img[2] = "url(https://….jpg)";
  break;
case 1:  /* 2月の画像 */
  bg_img[0] = "url(https://….jpg)";
  bg_img[1] = "url(https://….jpg)";
  bg_img[2] = "url(https://….jpg)";
  break;
case 2:  /* 3月の画像 */
  bg_img[0] = "url(https://….jpg)";
  bg_img[1] = "url(https://….jpg)";
  bg_img[2] = "url(https://….jpg)";
  break;
　　・
　　・
case 11:  /* 12月の画像 */
  bg_img[0] = "url(https://….jpg)";
  bg_img[1] = "url(https://….jpg)";
  bg_img[2] = "url(https://….jpg)";
  break;
}
n = Math.floor(Math.random()*bg_img.length);
document.getElementById('blog-title-inner').style.backgroundImage = bg_img[n];
<script>