var Img_no  = 2; //画像枚数
var Random_img = Math.floor (Math.random() * Img_no);

if	(Random_img == 0)	document.write ('<img src="snowmobile_red.gif" alt="赤スノーモビル">');
else if	(Random_img == 1)	document.write ('<img src="snowmobile_blue.gif" alt="青スノーモビル">');