Untitled

<img src="[acf field='image' post_id=''. $post_id .'']" />

<?php
function my_shortcode() {
    $output = '';
    $output.= '<img src="[acf field='image' post_id=''. $post_id .'']" />';
    return $output;
}

function my_shortcode() {
    ob_start();
    ?> <HTML> <img src="[acf field='image' post_id=''. $post_id .'']" /> <?php
    return ob_get_clean();
}