Drupal - How do I add a variable for a Twig template in my preprocess function?

I finally solved the problem, it turns out the problem was that I tried to add the variables in the wrong preprocessor. Editing the #theme works there, but to add variables I needed to preprocess the theme I set there:

function template_preprocess_imagegallery_format(&$vars) {
  template_preprocess_image_formatter($vars);
  $vars['image']['#theme'] = 'igimage';
}

function template_preprocess_igimage(&$vars) {
    $vars['thumbnail'] = ImageStyle::load('thumbnail')->buildUrl($vars['uri']);
    $vars['fullimage'] = file_create_url($vars['uri']);
}

Theme name : atvdirect

• create a file atvdirect.theme on root of your theme, aside atvdirect.info.yml
• add below code in atvdirect.theme file

• use {{ logopath }} in page.html.twig

  <?php
  function atvdirect_preprocess_page(&$variables) {
    $variables['logopath'] = '/'.drupal_get_path('theme','atvdirect') .'/logo.png';
  }
  ?>