Add featured images to next/prev links.

gistfile1.txt

if ( ! function_exists( 'generate_content_nav' ) ) :
/**
 * Display navigation to next/previous pages when applicable
 */
function generate_content_nav( $nav_id ) {

	global $wp_query, $post;

	// Don't print empty markup on single pages if there's nowhere to navigate.
	if ( is_single() ) {
		$previous = ( is_attachment() ) ? get_post( $post->post_parent ) : get_adjacent_post( false, '', true );
		$next = get_adjacent_post( false, '', false );

		if ( ! $next && ! $previous ) {
			return;
		}
	}

	// Don't print empty markup in archives if there's only one page.
	if ( $wp_query->max_num_pages < 2 && ( is_home() || is_archive() || is_search() ) ) {
		return;
	}

	$nav_class = ( is_single() ) ? 'post-navigation' : 'paging-navigation';
	$category_specific = apply_filters( 'generate_category_post_navigation', false );
	?>
	<nav id="<?php echo esc_attr( $nav_id ); ?>" class="<?php echo $nav_class; ?>">
		<h6 class="screen-reader-text"><?php _e( 'Post navigation', 'generatepress' ); ?></h6>

		<?php if ( is_single() ) : // navigation links for single posts 

			$prevPost = get_previous_post();
			$prevThumbnail = isset( $prevPost ) ? get_the_post_thumbnail( $prevPost->ID ) : '';
			$nextPost = get_previous_post();
			$nextThumbnail = isset( $nextPost ) ? get_the_post_thumbnail( $nextPost->ID ) : '';

			previous_post_link( '<div class="nav-previous"><span class="prev" title="' . __('Previous','generatepress') . '">%link</span></div>', $prevThumbnail . '%title', $category_specific );
			next_post_link( '<div class="nav-next"><span class="next" title="' . __('Next','generatepress') . '">%link</span></div>', $nextThumbnail . '%title', $category_specific );

		elseif ( is_home() || is_archive() || is_search() ) : // navigation links for home, archive, and search pages

			if ( get_next_posts_link() ) : ?>
				<div class="nav-previous"><span class="prev" title="<?php _e('Previous','generatepress');?>"><?php next_posts_link( __( 'Older posts', 'generatepress' ) ); ?></span></div>
			<?php endif;

			if ( get_previous_posts_link() ) : ?>
				<div class="nav-next"><span class="next" title="<?php _e('Next','generatepress');?>"><?php previous_posts_link( __( 'Newer posts', 'generatepress' ) ); ?></span></div>
			<?php endif;
			
			if ( function_exists( 'the_posts_pagination' ) ) {
				the_posts_pagination( array(
					'mid_size' => apply_filters( 'generate_pagination_mid_size', 1 ),
					'prev_text' => __( '&larr; Previous', 'generatepress' ),
					'next_text' => __( 'Next &rarr;', 'generatepress' ),
				) );
			}
			
			do_action('generate_paging_navigation');

		endif; ?>
	</nav><!-- #<?php echo esc_html( $nav_id ); ?> -->
	<?php
}
endif;