Jquery plugin para centrar verticalmente una imagen

gistfile1.js

$.fn.vCenter = function(){
    this.each(function(){
        parent_height = $(this).parent().height();
        image_height = $(this).height();
        top_margin = (parent_height - image_height)/2;
    });
 
    return this.each(function() {
        $(this).css( 'margin-top' , top_margin);
    });
 
}
$('SOME-ID-OR-CLASS').vCenter();