WordPress 投稿の前後記事へのリンクにアイキャッチ画像を使う ref: http://qiita.com/gatespace/items/34b9c71378bbfc20b6f0

my-post-navigation.css

.post-navigation .nav-previous a,
.post-navigation .nav-next a {
    display: block;
    color: white;
    background: #353c3f;
    background-repeat: no-repeat;
    background-position: center center;
    background-size: cover;
}
.post-navigation .nav-previous a:hover,
.post-navigation .nav-next a:hover {
    opacity: 0.9;
}
.post-navigation .nav-previous .link-text,
.post-navigation .nav-next .link-text {
    padding: 20px;
    display: block;
    rgba(0, 0, 0, 0.5);
}

my-post-navigation.php

<?php
if ( ! function_exists( 'my_post_navigation' ) ) :
/**
 * Display navigation to next/previous set of posts when applicable.
 */
function my_post_navigation() {

    $post_nav_html = '';
    // ダミー画像用意しておく
    $prev_post_img = get_template_directory_uri() . '/images/dummy.png';
    $next_post_img = get_template_directory_uri() . '/images/dummy.png';

    $prev_post = get_adjacent_post( false, '', true );
    if ( is_a( $prev_post, 'WP_Post' ) ) {
        if ( has_post_thumbnail( $prev_post->ID ) ) {
            $prev_post_img = wp_get_attachment_image_src( get_post_thumbnail_id( $prev_post->ID ), 'large' );
            $prev_post_img = $prev_post_img[0];
        }
        $post_nav_html .= '<div class="nav-previous">' . "\n";
        $post_nav_html .= '<a href="' . get_permalink( $prev_post->ID ) . '" rel="prev" style="background-image:url(' . $prev_post_img . ')"><span class="link-text"><span class="adjacent-text">Previous</span><br>' . get_the_title( $prev_post->ID ) . '</span></a>';
        $post_nav_html .= '</div>' . "\n";
    }

    $next_post = get_adjacent_post( false, '', false );
    if ( is_a( $next_post, 'WP_Post' ) ) {
        if ( has_post_thumbnail( $next_post->ID ) ) {
            $next_post_img = wp_get_attachment_image_src( get_post_thumbnail_id( $next_post->ID ), 'large' );
            $next_post_img = $next_post_img[0];
        }
        $post_nav_html .= '<div class="nav-previous">' . "\n";
        $post_nav_html .= '<a href="' . get_permalink( $next_post->ID ) . '" rel="prev" style="background-image:url(' . $next_post_img . ')"><span class="link-text"><span class="adjacent-text">Next</span><br>' . get_the_title( $next_post->ID ) . '</a></span>';
        $post_nav_html .= '</div>' . "\n";
    }

    if ( ! empty( $post_nav_html ) ) :
?>
    <nav class="navigation post-navigation" role="navigation">
        <h2 class="screen-reader-text"><?php esc_html_e( 'Post navigation', 'mytheme' ); ?></h2>
        <div class="nav-links">
        <?php echo $post_nav_html; ?>
        </div><!-- .links -->
    </nav><!-- .navigation -->
<?php
    endif;
}
endif;