Add options to Make Post Images Mandatory

<?php

add_action('save_post', 'evolution_check_thumbnail');
add_action('admin_notices', 'evolution_thumbnail_error');

function evolution_check_thumbnail($post_id) {
    // change to any custom post type
    if(get_post_type($post_id) != 'post')
        return;
    if ( !has_post_thumbnail( $post_id ) ) {
        // set a transient to show the users an admin message
        set_transient( "has_post_thumbnail", "no" );
        // unhook this function so it doesn't loop infinitely
        remove_action('save_post', 'evolution_check_thumbnail');
        // update the post set it to draft
        wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));
        add_action('save_post', 'evolution_check_thumbnail');
    } else {
        delete_transient( "has_post_thumbnail" );
    }
}
function evolution_thumbnail_error()
{
    // check if the transient is set, and display the error message
    if ( get_transient( "has_post_thumbnail" ) == "no" ) {
        echo "<div id='message' class='error'><p><strong>You have to assign a post image. Without a post image, the article can't be published.</strong></p></div>";
        delete_transient( "has_post_thumbnail" );
    }
}

https://www.noupe.com/wordpress/the-26-most-useful-and-most-functional-wordpress-snippets.html