img=new Array(7);
img[0] = "img/top/cog_img.jpg";
img[1] = "img/top/cog_img1.jpg";
img[2] = "img/top/cog_img2.jpg";
img[3] = "img/top/cog_img3.jpg";
img[4] = "img/top/cog_img4.jpg";
img[5] = "img/top/cog_img5.jpg";
img[6] = "img/top/cog_img6.jpg";

function topimg()
{
n = Math.floor(Math.random() * 7); // ←画像の枚数によって変える
document.write("<img src='",img[n],"' border=0>");
}