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!