I want to resized image of subcategory. for that,
I added this code in .phtml file.I'm getting wrong path for image

    <?php
        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $catId = 3;  //Parent Category ID
        $subCategory = $objectManager->create('Magento\Catalog\Model\Category')->load($catId);
        $subCats = $subCategory->getChildrenCategories();
        $_helper = $this->helper('Magento\Catalog\Helper\Output');
        $_imagehelper = $this->helper('Magento\Catalog\Helper\Image');
        $count = 0;
    ?>
    <div class="container homecat">
    <div class="row">
        <?php 
            foreach ($subCats as $subcat) {
            $_category = $objectManager->create('Magento\Catalog\Model\Category')->load($subcat->getId());
            $subcaturl = $subcat->getUrl();
            $_imgHtml = '';
            
            $subcaturl =  $_imagehelper->init($_category, 'image')->keepAspectRatio(TRUE)
                ->constrainOnly(TRUE)
                ->keepAspectRatio(TRUE)
                ->keepTransparency(TRUE)
                ->resize('256','128')
                ->getUrl();
            echo $subcaturl ."<br/>";
            $_imgHtml = '<img src="' . $subcaturl . '" />';
            $_imgHtml = $_helper->categoryAttribute($_category, $_imgHtml, 'image');
            echo $_imgHtml;
        } ?>
    </div>
    </div>

I am getting 

> http://Hostname/als2/pub/static/version1549014696/frontend/Theme/Name/en_US/Magento_Catalog/images/product/placeholder/.jpg