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

imageArray = new Array();
imageArray[ImageNum++] = image_dir + "glapola_1.jpg";
imageArray[ImageNum++] = image_dir + "glapola_2.jpg";
imageArray[ImageNum++] = image_dir + "glapola_3.jpg";
imageArray[ImageNum++] = image_dir + "glapola_4.jpg";
imageArray[ImageNum++] = image_dir + "glapola_5.jpg";
imageArray[ImageNum++] = image_dir + "glapola_6.jpg";
imageArray[ImageNum++] = image_dir + "glapola_7.jpg";
imageArray[ImageNum++] = image_dir + "glapola_8.jpg";
imageArray[ImageNum++] = image_dir + "glapola_9.jpg";
imageArray[ImageNum++] = image_dir + "glapola_10.jpg";
imageArray[ImageNum++] = image_dir + "glapola_11.jpg";
imageArray[ImageNum++] = image_dir + "glapola_12.jpg";
imageArray[ImageNum++] = image_dir + "glapola_13.jpg";
imageArray[ImageNum++] = image_dir + "glapola_14.jpg";
imageArray[ImageNum++] = image_dir + "glapola_15.jpg";
imageArray[ImageNum++] = image_dir + "glapola_16.jpg";
imageArray[ImageNum++] = image_dir + "glapola_17.jpg";
imageArray[ImageNum++] = image_dir + "glapola_18.jpg";
imageArray[ImageNum++] = image_dir + "glapola_19.jpg";
imageArray[ImageNum++] = image_dir + "glapola_20.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];
}