Just add this code to single.php, replace or comment the default function like the_post_navigation, get_previous_post, get_next_post, etc.
$prev_post = get_previous_post();
$prev_thumbnail = get_the_post_thumbnail( $prev_post->ID, 'large');
$next_post = get_next_post();
$next_thumbnail = get_the_post_thumbnail( $next_post->ID, 'large');
$args = array(
'prev_text' => $prev_thumbnail,
'next_text' => $next_thumbnail
);
the_post_navigation( $args );Easy!