Tweak next_post_link for it to include thumbnail

If you have a look at what next_post_link you’ll see that it takes a parameter $link as the second argument:

next_post_link( $format, $link, $in_same_term = false, $excluded_terms="", $taxonomy = 'category' );

Your parameter for this currently doesn’t include the post thumbnail:

_x( '%title <span class="meta-nav">&rarr;</span>', 'Next post link', 'nicosite' )

So we need to add it like this:

$nextPost = get_next_post(true); // You are aware that this only checks for next posts in the same term, right? 
if(!empty($nextPost) ){ //This is to check if there is a next post
    $nextthumbnail = get_the_post_thumbnail($nextPost->ID, array(300,150) );
}else{
    $nextthumbnail="";
}
next_post_link( '<div class="nav-next">%link</div>', $nextthumbnail . _x( '%title <span class="meta-nav">&rarr;</span>', 'Next post link', nicosite' ) );