var anim_nb = 37;
var anim_speed = 10000;
var anim_index = Math.floor(new Date().getTime() / anim_speed) % anim_nb;

function setImageAnim(n) {
	var e = document.getElementById('topanim');
	e.style.backgroundImage = 'url(images/top/im-' + n + '.jpg)';
}

setImageAnim(anim_index);
setInterval( function() {
	anim_index++;
	anim_index%=anim_nb;
	setImageAnim(anim_index);
}, anim_speed);