var zvacsit = 1;
var Blava = new Image(484, 322);
var ustav = new Image(484, 322);
var zoom_in  = new Image(80, 20);
var zoom_out = new Image(80, 20);

Blava.src = "images/blava.jpg";
ustav.src = "images/sav.jpg";
zoom_in.src ="images/zoom_in.jpg";
zoom_out.src="images/zoom_out.jpg";

function mierka()
{
  i = previous_images;

  if (zvacsit == 1) {
    document.images[i].src   = ustav.src;
    document.images[i+1].src = zoom_out.src;
    zvacsit = 0;
  }
  else if (zvacsit == 0) {
    document.images[i].src   = Blava.src;
    document.images[i+1].src = zoom_in.src;
    zvacsit = 1;
  }
}