Word Press: Why are my images in single.php are messed up?

When uploading images to my wp articles they are not displayed properly. (Please take a look at attached photograph: The images have not the same dimensions even though I set them within admin panel to “full width”)

This is the - I think - relevant code in functions.php:

function myBlog_features() {
  add_theme_support('title-tag');
  add_theme_support('post-thumbnails');
  add_image_size('landscape', 400, 260, true);
  add_image_size('portrait', 480, 650, true);

  set_post_thumbnail_size(700, 700);
}
add_action('after_setup_theme', 'myBlog_features');

Do I have to use a different property than add_image_size? PLease not: It is not about the featured image, the Title Picture of a blog article. My issues are related to images within the blog posts.

This is the code in single.php

 <section class="main-wrapper">
<?php 
      while(have_posts()) {
    the_post(); ?>
<article class="article main-article_single">
    <?php
if ( has_post_thumbnail() ) { 
the_post_thumbnail( 'full' );
}
?>  
    <h1 class="article-headline">
            <?php the_title() ?>
    </h1>
    <p class="article-body">
        <?php the_content(); ?>
    </p>
</article>
    </section>

Do I have to add here sth. like <?php if image …?> ?
But what would it be?

As always: Best and thank you!

Sven