$.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";
    }
  });
};