function markCurrentPage(curPage, levels, filename) {
	var path = '';
	for (var i = 0; i < levels; i++) {
		path = path + "../";
	}
	path = path + "_images/";
	
	if (filename) {
		document[curPage].src = path + filename;
	} else {
		// default image
		document[curPage].src = path + "arrow-inc.gif";
	}
}