This is a basic WP query thatreturns all posts that have a featured image.

  'post_type'      => 'articles', // your post type
  'posts_per_page' => -1,         // all posts
  'post_status'    => 'publish',
  'meta_query'     => [
    [
      'key'     => '_thumbnail_id',
      'compare' => 'EXISTS',
    ],
  ],