Magento – Get URL of the static content folder (for backend)

You can try to use base folder inside View if you want to have the one file in frontend and backend both:

-app
   -code
       -Roland
           -HelloWorld
               -view
                   -base
                       -web
                          -image.png

The next folder, view, is the area folder. Areas area a way to split individual Magento 2 applications into different areas of functionality, based on the URL. i.e. The cart application is Magento’s frontend area, the backend admin console is the adminhtml area.

So what’s the base area? This is one of those places where Magento 2 has improved on Magento 1 — the base area is a special folder that will allow you to serve your files from either the frontend or adminhtml areas. We’ll talk more about this below.

You can find more info here (source).

You can check in details by click, Resize Custom Image in Magento 2

Inside Block file keep below code,

   protected $_filesystem ;
   protected $_imageFactory;
   public function __construct(            
        \Magento\Framework\Filesystem $filesystem,         
        \Magento\Framework\Image\AdapterFactory $imageFactory         
        ) {         
        $this->_filesystem = $filesystem;               
        $this->_imageFactory = $imageFactory;         
        }

    // pass imagename, width and height
    public function resize($image, $width = null, $height = null)
    {
        $absolutePath = $this->_filesystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA)->getAbsolutePath('custom_module/posts/').$image;
        if (!file_exists($absolutePath)) return false;
        $imageResized = $this->_filesystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA)->getAbsolutePath('resized/'.$width.'/').$image;
        if (!file_exists($imageResized)) { // Only resize image if not already exists.
            //create image factory...
            $imageResize = $this->_imageFactory->create();         
            $imageResize->open($absolutePath);
            $imageResize->constrainOnly(TRUE);         
            $imageResize->keepTransparency(TRUE);         
            $imageResize->keepFrame(FALSE);         
            $imageResize->keepAspectRatio(TRUE);         
            $imageResize->resize($width,$height);  
            //destination folder                
            $destination = $imageResized ;    
            //save image      
            $imageResize->save($destination);         
        } 
        $resizedURL = $this->_storeManager->getStore()->getBaseUrl(\Magento\Framework\UrlInterface::URL_TYPE_MEDIA).'resized/'.$width.'/'.$image;
        return $resizedURL;
  } 

Now Call inside phtml file,

$block->resize('test.jpg',500,400);