var speed = 20;
var sizeX = 560;
var sizeY = 420;
var stepX = 0;
var stepY = 0;
var steps = 150;
var imgObj;
function ustaw()
{
  imgObj = document.getElementById('obrazekId');
  stepX = Math.ceil(sizeX / steps);
  stepY = Math.ceil(sizeY / steps);
}
function resize()
{
  currentW = imgObj.width;
  currentH = imgObj.height;
  if ((currentW < sizeX) && (currentH < sizeY)){
    imgObj.width = currentW + stepX;
    imgObj.height = currentH + stepY;
    setTimeout("resize()", speed);
  }
}