function getRandomIndex(max) {
        var randomNum = Math.random();
        randomNum = parseInt(randomNum * max);
        if(isNaN(randomNum) || randomNum == 0) randomNum = 1; // for Netscape
        return randomNum;
}
function writeBanner(){
BannerRandom = getRandomIndex(7);
var image = new Array(7);
image[1] = 'obj_topimage_01';
image[2] = 'obj_topimage_02';
image[3] = 'obj_topimage_03';
image[4] = 'obj_topimage_04';
image[5] = 'obj_topimage_05';
image[6] = 'obj_topimage_06';
image[7] = 'obj_topimage_07';
 
var imagetype = image[BannerRandom];
document.write ('<img src=images/banner/'+image[BannerRandom]+'.jpg border=0>');
}