tag = new Array();
tag[0]='<img src="img/01.jpg" alt="banner" class="mt-image-none"/>';
tag[1]='<img src="img/02.jpg" alt="banner" class="mt-image-none"/>';
tag[2]='<img src="img/03.jpg" alt="banner" class="mt-image-none"/>';
tag[3]='<img src="img/04.jpg" alt="banner" class="mt-image-none"/>';
i　=　Math.floor(Math.random()*tag.length);
document.write(tag[i]);