Render image field from node in page.html.twig
I would say that the Twig-ish way is to create a custom filter. The Drupal-ish way is to use a hook_preprocessor. The general idea of the MVC rewrite is to keep logic out of the templates. I couldn't see how to add Twig filters in Drupal 8 (anyone?) so I'll go with preprocessor.

Drupal 8 preprocessor

In your theme's template.php:

function mytheme_preprocess_page(&amp;$variables) {
  if ( $variables['node'] ) {
    $node = $variables['node'];
    // Optionally limit by bundle type. How do you get a bundle in D8?
    // if ( $bundle != 'blog' ) return;
    // Get your image URI.
    $header_image = $node-&gt;get('headerimage')-&gt;value;
    // Just guessing here: not sure what to do but it may involve styles.
    $header_uri = $header_image-&gt;getFileUri();
    // Override the variable that normally stores the header image.
    // Not sure what that would be. Look in page.html.twig
    $variables['headerimage'] = $header_uri;
  }
}