//支持外链缩略图
if ( function_exists('add_theme_support') )
 add_theme_support('post-thumbnails');
function catch_first_image() 
{
	global $post, $posts;$first_img = '';
	ob_start();
	ob_end_clean();
	$output = preg_match_all('/<img.+src=['"]([^'"]+)['"].*>/i', $post->post_content, $matches);
	$first_img = $matches [1] [0];
	//判断图片是否过小
	if(!empty($first_img))
	{
		$image_size = getimagesize($first_img);
		$image_width = $image_size[0];
	}
	//如果第一张图不存在或过小，则返回随机图片
	if(empty($first_img) || $image_width<50){
		$first_img = '';
		//从2张图中随机选择，可根据自己的图片数量设置
		$random = mt_rand(1, 2);
		echo get_bloginfo ( 'stylesheet_directory' );
		echo '/images/random/'.$random.'.JPG';
		}
  return $first_img;
}

<?php echo catch_first_image(); ?>