get path of uploaded picture for custom parallax effect
You'll have to set a $src var from the corresponding preprocess hook. 

function myModuleOrTheme_preprocess_block(&amp;$variables){

if(isset($variables['elements']['content']['field_parralaxBgImage']))
//or any other way to test if you are in the right block

   //Get the Image id : 
   $imgListItems = $variables['elements']['content']['field_testimages']['#items'];
   $imgId = $imgListItems-&gt;getValue()[0]['target_id']; 
   //load the entity :
   $image = \Drupal\file\Entity\File::load($imageId);

  // now you can simply return the original image's url : 
  $variables['parralaxBgSrc'] = image-&gt;url();

  //or return a styled image's url (croped or resized) with a image style configured in the BO
  $imageStyle = \Drupal\image\Entity\ImageStyle::load('imageStyle_id');
  $variables['parralaxBgSrc'] = $imageStyle-&gt;buildUrl($image-&gt;getFileUri());

}


After that, you'll be able to use it in your twig tpl : 

 &lt;div class="parallax-window" data-parallax="scroll" data-image-src="{{$parralaxBgSrc}}"&gt; 


(Dont forgot to flush your cache after adding the hook) 

Et voila! :)