How can I add a horizontal image in between posts?

You can add a filter to the content to display the image after every post.

function funny_cat_filter( $content ) {
    $content .= '<img src="https://funnylolcats.com/lolcatz.jpg" />';

   return $content;

}

add_filter( 'the_content', 'funny_cat_filter' );

so this code goes where then? you said to the content which makes me think add it html to each post but that doesn’t seem right to me so i’m thinking you mean to put it somewhere else

it’s a function – which would go in functions.php!

lol thank you!

No problem! It seems obvious to some of us who’ve done this a bit – but when you are new to this stuff, it’s not so obvious!!