code: private function resizeImage($fileName)
      {
          list($width, $height, $mode) = $this->getResizeImages();
          if ($this->getShowImages() && ($width || $height || $mode)) {
              if ($this->imageFactory) {
                  $image = $this->imageFactory->create(
                      $this->rootDir . '/' . $fileName,
                      [$width, $height, $mode]
                  );
  
                  return $image->getUrl($this->rootDir);
              }
  
              $event = new ResizeImageEvent($fileName, $width, $height, $mode);
              $this->dispatcher->dispatch(ContaoEvents::IMAGE_RESIZE, $event);
              return $event->getResultImage();
          }
  
          return $fileName;
      }
docs: Resize the image if needed.
  
  @param string $fileName The file to resize.
  
  @return null|string