Related Posts without plugin http://www.wppicker.com/how-to-add-responsive-related-posts-without-plugin/

wpp-related-posts.php

<?php
/*
Outputs related posts with thumbnail -------------------------------*/
 
 function wpp_related_posts($content) {
     
    if ( is_single ( ) ) {
         
        global $post;
 
	$numposts = 4; /*this line decides the number of related posts to include*/
        $count = 0;
        $postIDs = array( $post->ID );
        $related = '';
        $tags = wp_get_post_tags( $post->ID );
        $cats = wp_get_post_categories( $post->ID );
         
        if ( $tags ) {
            foreach ( $tags as $tag ) {
                $tagID[] = $tag->term_id;
            }
            $args = array(
                'tag__in'               => $tagID,
                'post__not_in'          => $postIDs,
                'showposts'             => $numposts,
                'ignore_sticky_posts'   => 1,
                'tax_query'             => array(
                    array(
                                        'taxonomy'  => 'post_format',
                                        'field'     => 'slug',
                                        'terms'     => array( 
                                            'post-format-link', 
                                            'post-format-status', 
                                            'post-format-aside', 
                                            'post-format-quote'
                                            ),
                                        'operator'  => 'NOT IN'
                    )
                )
            );
			$tag_query = new WP_Query( $args );
             
            if ( $tag_query->have_posts() ) {
                while ( $tag_query->have_posts() ) {
                    $tag_query->the_post();
                    			/*set featured image or fallback image for related posts*/
					$img = has_post_thumbnail() ? get_the_post_thumbnail( $post_id, 'thumbnail' ) : '<img src="' . get_bloginfo( 'stylesheet_directory' ) . '/images/related.jpg" alt="' . get_the_title() . '" />';
                    
					$related .= '<li><a href="' . get_permalink() . '" rel="bookmark" title="' . get_the_title() . '">' . $img . get_the_title() . '</a></li>';
                    
					$postIDs[] = $post->ID;
					$count++;
                }
            }
        }
 
        if ( $count <= ($numposts - 1) ) {
             
            $catIDs = array( );
            foreach ( $cats as $cat ) {
                $catIDs[] = $cat;
            }
             
            $showposts = $numposts - $count;
 
            $args = array(
                'category__in'          => $catIDs,
                'post__not_in'          => $postIDs,
                'showposts'             => $showposts,
                'ignore_sticky_posts'   => 1,
                'orderby'               => 'rand',
                'tax_query'             => array(
                                    array(
                                        'taxonomy'  => 'post_format',
                                        'field'     => 'slug',
                                        'terms'     => array( 
                                            'post-format-link', 
                                            'post-format-status', 
                                            'post-format-aside', 
                                            'post-format-quote' ),
                                        'operator' => 'NOT IN'
                                    )
                )
            );
 
            $cat_query = new WP_Query( $args );
            
            if ( $cat_query->have_posts() ) {
                 
                while ( $cat_query->have_posts() ) {
                     
                    $cat_query->the_post();
                    
 		    /*set featured image or fallback image for related posts*/	
                    $img = has_post_thumbnail() ? get_the_post_thumbnail( $post_id, 'thumbnail' ) : '<img src="' . get_bloginfo( 'stylesheet_directory' ) . '/images/related.jpg" alt="' . get_the_title() . '" />';
 
                    $related .= '<li><a href="' . get_permalink() . '" rel="bookmark" title="' . get_the_title() . '">' . $img . get_the_title() . '</a></li>';
                }
            }
        }
 
        if ( $related ) {
             
            $content=$content. '<div class="wpp-related-posts"><h3 class="wpp-related-title">Related Posts</h3><ul class="wpp-related-list">'.$related.'</ul></div><div class="wpp-clear"></div>';
        }
         
        wp_reset_query();
		
    }
	return $content ;
} 

?>