uploaded custom category image attribute but after saving the category,the image is not showing.

[Abin jose]

i created a controller for uploading image. and added a composer.json file 

// custom controller 

<?php
namespace Ventor\Theme\Controller\Adminhtml\Category\Thumbnailimage;

use Magento\Framework\Controller\ResultFactory;

/**
 * Class Upload
 */
class Upload extends \Magento\Backend\App\Action
{
    protected $baseTmpPath;
    protected $imageUploader;
    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \Magento\Catalog\Model\ImageUploader $imageUploader
    ) {
        $this->imageUploader = $imageUploader;
        parent::__construct($context);

    }
    public function execute() {

        try {
            $result = $this->imageUploader->saveFileToTmpDir('thumbnail');
            $result['cookie'] = [
                'name' => $this->_getSession()->getName(),
                'value' => $this->_getSession()->getSessionId(),
                'lifetime' => $this->_getSession()->getCookieLifetime(),
                'path' => $this->_getSession()->getCookiePath(),
                'domain' => $this->_getSession()->getCookieDomain(),
            ];
        } catch (\Exception $e) {
            $result = ['error' => $e->getMessage(), 'errorcode' => $e->getCode()];
        }
        return $this->resultFactory->create(ResultFactory::TYPE_JSON)->setData($result);
    }
}

//custom composer.json file
{
    "name": "categorycarousel/magento2-category-image",
    "description": "Add custom category image attribute",
    "require": {
        "php": "~5.5.0|~5.6.0"
    },
    "type": "magento2-module",
    "version": "1.0.0",
    "license": [
        "OSL-3.0",
        "AFL-3.0"
    ],
    "extra": {
        "map": [
            [
                "*",
                "Ventor/Theme"
            ]
        ]
    }
}