$.fn.center = function(f) {
	return this.each(function(){
		var p = this.parentNode;
		if ( $.css(p,"position") == 'static' )
			p.style.position = 'relative';

		var s = this.style;
		s.position = 'absolute';
		if(!f || f == "horizontal") { 
			if(((parseInt($.css(p,"width")) - parseInt($.css(this,"width")))/2) > 0)
				s.left = ((parseInt($.css(p,"width")) - parseInt($.css(this,"width")))/2) + "px";
			else
				s.left = "0px";
		}
		if(!f || f == "vertical") {	
			if(((parseInt($.css(p,"height")) - parseInt($.css(this,"height")))/2) > 0)						
				s.top = ((parseInt($.css(p,"height")) - parseInt($.css(this,"height")))/2) + "px";
			else
				s.top = "0px";
		}
	});
};