[ruslan_mart]

var d = 0,
	z = 0,
	gallery = document.getElementById('gallery').style;

function right(len) {
	d = setInterval(function() {
		if(z == -len) clearInterval(d);
		else gallery.left = (z -= 5) + 'px';
	}, 10);
}

<img onmouseover="right(50)" onmouseout="clearInterval(d)" />