Author picture in the beginning of posts - How to add?

Hi team,

I'm trying to add the author picture to my posts, as per this thread: https://generatepress.com/forums/topic/add-author-photo-for-each-post-in-blog-home/

But it's not working for me.

All I want is for the author picture to appear next to the author name in the beginning of every post.

My coding skills are that of a pensioner who takes courses to learn Microsoft Word. Almost.

In the thread above you guys instructed to add this:

add_filter( 'generate_post_author_output', 'tu_add_author_gravatar' );
function tu_add_author_gravatar() {
printf( ' <span class="byline">%1$s</span>',
sprintf( '<span class="author vcard" itemtype="http://schema.org/Person" itemscope="itemscope" itemprop="author">%1$s %5$s<span class="author-name" itemprop="name">%4$s</span></span>',
__( 'by','generatepress'),
esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ),
esc_attr( sprintf( __( 'View all posts by %s', 'generatepress' ), get_the_author() ) ),
esc_html( get_the_author() ),
get_avatar( get_the_author_meta( 'ID' ) )
)
);
}

I've tried adding that to my function file (child's theme), but that didn't work. When I did that, my site had a big white bar appearing at the top of the screen, containing some of the above codes, in black text.

I also tried adding it under "Extra CSS", but Wordpress then gave me a bunch of warnings. I clicked the "update anyway" and published. But no success, nothing happened.

If I want to add a round gravatar picture for every author in their posts, how do I do it? I would be very grateful for step-by-step instructions on how to do it. What I wish to have is exactly what is described in the other thread.

Thank you very much in advance and have a good weekend!

Best regards,
Anton