How to add a custom image field to a category in Magento?

Step:1. At first run this code  
 $installer = new Mage_Eav_Model_Entity_Setup('core_setup');
    $installer->startSetup();
    $attribute  = array(
    'type'          =>  'text',
    'label'         =>  'Banner Image',
    'input'         =>  'image',
    'global'        =>  Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
    'visible'       =>  true,
    'required'      =>  false,
    'backend'       => 'catalog/category_attribute_backend_image',
    'user_defined'  =>  true,
    'default'       =>  "",
    'group'         =>  "General Information"
    );
    $installer->addAttribute('catalog_category', 'banner_image', $attribute);
    $installer->endSetup();



Step 2:
Copy Category.php file from app/code/core/Mage/Catalog/Model/Category.php
to
 app/code/local/Mage/Catalog/Model/Category.php

Add this code line 480
 public function getBannerImageUrl()
    {
        $url = false;
        if ($image = $this->getBannerImage()) {
            $url = Mage::getBaseUrl('media').'catalog/category/'.$image;
        }
        return $url;
    }