Magento Category image resize

1-Copy category folder from base/default/template/catalog/category and paste it in your theme before modification.
2-copy and paste the following code in your-theme/default/template/catalog/category/view.phtml

$_file_name = $_category->getThumbnail();             
$_media_dir = Mage::getBaseDir('media') . DS . 'catalog' . DS . 'category' . DS;
$cache_dir = $_media_dir . 'resize' . DS;

if (file_exists($cache_dir . $_file_name)) {
$_categoryImg =Mage::getBaseUrl('media') .  'catalog' . DS . 'category' . DS . 'resize' . DS . $_file_name;
} elseif (file_exists($_media_dir . $_file_name)) {
if (!is_dir($cache_dir)) {
mkdir($cache_dir);
}

$_image = new Varien_Image($_media_dir . $_file_name);
$_image->constrainOnly(true);
$_image->keepAspectRatio(false);
$_image->keepFrame(false);
$_image->keepTransparency(true);
$_image->resize(500, 500); // change image height, width
$_image->save($cache_dir . $_file_name);
$_categoryImg = Mage::getBaseUrl('media') . 'catalog' . DS . 'category' . DS . 'resize' . DS . $_file_name;
}
//echo  $_categoryImg ; // display resize category thumbnail imagename