Appending code to the_content

You are looking to filter the_content.

In your theme’s functions.php or in a plugin:

function wpse74288_content_filter( $content ) {
    /* save field into variable, such as to call get_field only once
     * not required, but more efficient */
    $who_specialises = get_field('who_specialises');
    /* append to content on condition */
    if ( in_array( 'frazer-barton', $who_specialises ) ) {
        /* use correct img src attributes, shortened only for representation here */
        $content .= '<img src="http://andersonlloyd...images/example-thumb.jpg" />';
    } elseif ( in_array( 'sarah-simmers', $who_specialises ) ) {
        $content .= '<img src="http://andersonlloyd.../images/example2-thumb.jpg" />';
    }
    /* return the content, whether appending happened or not */
    return $content;
}
add_filter( 'the_content', 'wpse74288_content_filter' );

As an aside, this question is probably the dupe of a dupe of a dupe [+ n dupes], had you known what to search for.