Access a field from page.html.twig template
The page title has its own preprocess hook. You may want to use hook_preprocess_HOOK() to add your image to the variables. As it is using the page-title.html.twig template, the theme registry HOOK is called 'page_title'.

Following is an excerpt from a theme's page title preprocess function of a D8 theme that is adding the URI of an image with an image style applied along with the page title. Maybe you find it useful as inspiration for your use case:

use Drupal\image\Entity\ImageStyle;

/**
 * Implements hook_preprocess_HOOK() for page title images.
 */
function mytheme_preprocess_page_title(array &amp;$variables) {
  $image = '';

  // Load the image entity from current route.
  if (
    !empty($node = \Drupal::routeMatch()-&gt;getParameter('node')) &amp;&amp;
    $node-&gt;getType() == 'page' &amp;&amp;
    $node-&gt;hasField('field_image') &amp;&amp;
    $image_entity = $node-&gt;field_image-&gt;entity
  ) {
    $style = ImageStyle::load('title_image');
    $image = ($style-&gt;buildUri($image_entity-&gt;getFileUri());
  }

  $variables['image'] = $image;
}