add_filter( 'fl_theme_builder_post_nav', function( $args ) {
$next_post = get_next_post();
$previous_post = get_previous_post();
if ( isset( $next_post->ID ) ) {
$args['next_text'] = '<span class="post-title">%title</span>' . get_the_post_thumbnail( $next_post->ID, 'thumbnail' );
}
if ( isset( $previous_post->ID ) ) {
$args['prev_text'] = '<span class="post-title">%title</span>' . get_the_post_thumbnail( $previous_post->ID, 'thumbnail' );
}
return $args;
} );