//additional properties for jQuery object
$(document).ready(function(){
   //align element in the middle of the screen
   $.fn.alignCenter = function() {
      //get margin left
      var marginLeft = Math.max(40, parseInt($(window).width()/2 - $(this).width()/2)) + 'px';
      //get margin top
      var marginTop = Math.max(40, parseInt($(window).height()/2 - $(this).height()/2)) + 'px';
      //return updated element
      return $(this).css({'margin-left':marginLeft, 'margin-top':marginTop});
   };

});