gistfile1.txt

var img = document.getElementById('catty');
img.style.position="absolute";
img.style.left = "0px";
max = document.width - img.width;

function move_right(){
  if(parseInt(img.style.left, 10) <= max){
		img.style.left = (parseInt(img.style.left, 10) + 15) + 'px';
	} else if(parseInt(img.style.left, 10) >= max) {
		var timer = setInterval(move_left, 100);
	}
}

function move_left(){
	if(parseInt(img.style.left, 10) >= 0){
		img.style.left = (parseInt(img.style.left, 10) - 15) + 'px';
	} else if(parseInt(img.style.left, 10) <= 0) {
		var timer = setInterval(move_right, 100);
	}
}

var timer = setInterval(move_right, 100);