var image_dir = "images/";
var ImageNum = 0;
var TotalNum = 0;

imageArray = new Array();
imageArray[ImageNum++] = image_dir + "galbum50_1.jpg";
imageArray[ImageNum++] = image_dir + "galbum50_3.jpg";

TotalNum = ImageNum;
ImageNum = -1;

function AvanzaImage()
{
   ImageNum++;
   if (ImageNum == TotalNum)
     ImageNum=0;
   document.mainimage.src = imageArray[ImageNum];
}

function RetrocedeImage()
{
   ImageNum--;
   if (ImageNum == -1)
     ImageNum= TotalNum - 1;
   document.mainimage.src = imageArray[ImageNum];
}