wordpress随机调用带缩略图的相关文章的方法，话不多说开始吧。

//添加特色缩略图支持
if ( function_exists('add_theme_support') )add_theme_support('post-thumbnails');
 
//输出缩略图地址 From wpdaxue.com
function post_thumbnail_src(){
    global $post;
	if( $values = get_post_custom_values("thumb") ) {	//输出自定义域图片地址
		$values = get_post_custom_values("thumb");
		$post_thumbnail_src = $values [0];
	} elseif( has_post_thumbnail() ){    //如果有特色缩略图，则输出缩略图地址
        $thumbnail_src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID),'full');
		$post_thumbnail_src = $thumbnail_src [0];
    } else {
		$post_thumbnail_src = '';
		ob_start();
		ob_end_clean();
		$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
		$post_thumbnail_src = $matches [1] [0];   //获取该图片 src
		if(empty($post_thumbnail_src)){	//如果日志中没有图片，则显示随机图片
			$random = mt_rand(1, 10);
			echo get_bloginfo('template_url');
			echo '/images/pic/'.$random.'.jpg';
			//如果日志中没有图片，则显示默认图片
			//echo '/images/default_thumb.jpg';
		}
	};
	echo $post_thumbnail_src;
}

<h3>相关文章</h3>
<ul class="related_img">
<?php
$post_num = 4;
$exclude_id = $post->ID;
$posttags = get_the_tags(); $i = 0;
if ( $posttags ) {
 $tags = ''; foreach ( $posttags as $tag ) $tags .= $tag->term_id . ',';
 $args = array(
 'post_status' => 'publish',
 'tag__in' => explode(',', $tags),
 'post__not_in' => explode(',', $exclude_id),
 'caller_get_posts' => 1,
 'orderby' => 'rand',
 'posts_per_page' => $post_num
 );
 query_posts($args);
 while( have_posts() ) { the_post(); ?>
 <li class="related_box" >
 <div class="r_pic">
 <a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>" target="_blank">
 <img src="<?php echo post_thumbnail_src(); ?>" alt="<?php the_title(); ?>" class="thumbnail" />
 </a>
 </div>
 <div class="r_title"><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>" target="_blank" rel="bookmark"><?php the_title(); ?></a></div>
 </li>
 <?php
 $exclude_id .= ',' . $post->ID; $i ++;
 } wp_reset_query();
}
if ( $i < $post_num ) {
 $cats = ''; foreach ( get_the_category() as $cat ) $cats .= $cat->cat_ID . ',';
 $args = array(
 'category__in' => explode(',', $cats),
 'post__not_in' => explode(',', $exclude_id),
 'caller_get_posts' => 1,
 'orderby' => 'rand',
 'posts_per_page' => $post_num - $i
 );
 query_posts($args);
 while( have_posts() ) { the_post(); ?>
 <li class="related_box" >
 <div class="r_pic">
 <a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>" target="_blank">
 <img src="<?php echo post_thumbnail_src(); ?>" alt="<?php the_title(); ?>" class="thumbnail" />
 </a>
 </div>
 <div class="r_title"><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>" target="_blank" rel="bookmark"><?php echo mb_strimwidth(get_the_title(), 0, 20, '...'); ?></a></div>
 </li>
 <?php $i++;
 } wp_reset_query();
}
if ( $i == 0 ) echo '<div class="r_title">没有相关文章!</div>';
?>
</ul>

.related_posts{margin-top:5px;} .related_img{width:600px;height:210px;} .related_box{float:left;overflow:hidden;margin-top:5px;width:148px;border-right:1px #eee solid} .related_box:hover{background:#f9f9f9} .related_box .r_title{width:auto;height:72px;font-weight:400;font-size:14px;margin:0 10px;overflow:hidden;} .related_box .r_pic{margin:6px} .related_box .r_pic img{width:130px;height:100px;border:1px solid #e1e1e1;background:#fff;padding:2px}

注释：

<?php echo mb_strimwidth(get_the_title(), 0, 20, '...'); ?>