[Synov_son]

Решил проблему:

function switch_to_slide(next){
	var images = $('#slideshow img').length;
	var id = get_id(images);
	$('#slideshow #thumbs span#'+next).addClass('active');
	$('#slideshow img#'+next).fadeIn(1500, 'easeOutQuart');
	$('#slideshow #thumbs span#'+id).removeAttr('class');
	$('#slideshow img#'+id).fadeOut(1500, 'easeOutQuart', function(){ play_slideshow() });
}