Using wordpress functions to add featured image to block

Hi I'm a bit new with adding custom functions in my code but I'm building a blog and in the every post page has a button where it says previous or next article. I wanted to use that pre existing function but display a block with the featured image for the each previous/next post.

I'm using Radcliffe theme and the arrows are in the bottom of the page:

http://ift.tt/1Kts5Hz

This is the part of the code I wanna add the "block"

Thank you

                    <div class="post-nav">

                    <?php
                    $next_post = get_next_post();
                    if (!empty( $next_post )): ?>

                        <p class="post-nav-next">

                            <a title="<?php _e('Next post:', 'radcliffe'); echo ' ' . get_the_title($next_post); ?>" href="<?php echo get_permalink( $next_post->ID ); ?>"><?php echo get_the_title($next_post); ?> </a>

                        </p>

                    <?php endif; ?>

                    <?php
                    $prev_post = get_previous_post();
                    if (!empty( $prev_post )): ?>


                        <p class="post-nav-prev">

                        <a title="<?php _e('Previous post:', 'radcliffe'); echo ' ' . get_the_title($prev_post); ?>" href="<?php echo get_permalink( $prev_post->ID ); ?>"><?php echo get_the_title($prev_post); ?> </a>

                        </p>

                    <?php endif; ?>