Upload Image to Blog Post

In the backend I can add images to my blog post. But they do not appear in the front-end or, if they appear, their size is messed up.
I use:
<?php the_post_thumbnail(array(700, 700) ); ?>
But this is only for the featured image

I’ve found sth. which looked like a solution, but I got
Warning: count(): Parameter must be an array or an object that implements Countable in /Applications/MAMP/htdocs/blog2020/wp-includes/post-template.php on line 317

So my question is: could anyone provide me with the necessary code I’ve to place in my single.php so that I can upload images in my blog post?

I am relatively new to WP-Dev and I’ve no idea, what to do. I am a bit lost in the Codex and the Developer Handbook…

My code is here:
https://github.com/Sven72/blogProject2020

Many thanks in advance! Best