How to add a HTML element in ADMIN edit post window?

This will work. It will add a meta box below the content editor window which outputs anything you put in the get_preview_image() function. I’ve included the $post argument so you can grab info from the post.

If you need this to be on a custom post type instead of posts, edit where it says ‘post’ in the image_preview_add_meta_box() function.

add_action( 'some_image_preview', 'get_preview_image', 10, 1 );

function image_preview_add_meta_box() {
    add_meta_box(
        'image_preview-image-preview',
        __( 'Image Preview', 'image_preview' ),
        'image_preview_html',
        'post',
        'normal',
        'high'
    );
}
add_action( 'add_meta_boxes', 'image_preview_add_meta_box' );

function image_preview_html( $post ) {
    wp_nonce_field( '_image_preview_nonce', 'image_preview_nonce' );
    do_action( 'some_image_preview', $post );
}

function get_preview_image( $post ) {
    echo 'This is cool!';
}