adding custom fields to next and previous post link

Here’s the code for what you’re trying to do (accessing post custom value):

<?php    
global $post;

    $prev_post = get_adjacent_post();
    $next_post = get_adjacent_post( false, '', false );
    $prev_post_id = $prev_post->ID;
    $next_post_id = $next_post->ID;
    // this should, according to your code above, be the http://example.com/your/img.jpg string
    $prev_img_path = get_post_custom_values( 'your_key_name', $prev_post_id );
    $next_img_path = get_post_custom_values( 'your_key_name', $next_post_id );
    $prev_def_path = get_bloginfo('stylesheet_directory').'/default_next_img_inside_theme_folder.jpg';
    $next_def_path = get_bloginfo('stylesheet_directory').'/default_prev_img_inside_theme_folder.jpg';

    echo '<img src="'.!empty($prev_img_path) ? $prev_img_path : $prev_def_path.'" alt="Permalink to previous post" />';
    echo '<img src="'.!empty($next_def_path) ? $next_def_path : $next_img_path.'" alt="Permalink to next post" />';
?>