Magento: Add custom attribute field in category

Step 1 :

Write below code in head.phtml after run one time remove the code.

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');
$setup->addAttribute('catalog_category', 'your_image', array(
    'group'         => 'General Information',
    'input'         => 'image',
    'type'          => 'varchar',
    'label'         => 'Your Image',
    'backend'       => 'catalog/category_attribute_backend_image',
    'visible'       => 1,
    'required'      => 0,
    'user_defined'  => 1,
    'global'        => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
));

Step 2 :

Override the Category model, copy Category.php file from app/code/core/Mage/Catalog/Model/Category.php to app/code/local/Mage/Catalog/Model/Category.php

Step 3 :

Add the following function in Category Model file

/* Return category image url
* @return string
* @author Sushant Vishwas
*/
public function getYourImageUrl() {
   $url = false;
   if ($image = $this->getYourImage()) {
       $url = Mage::getBaseUrl('media').'catalog/category/'.$image;
   }
   return $url;
}