修改nginx网站配置文件：

location ~ .*\.(gif|jpg|jpeg|png|bmp|swf)$
{
    valid_referers none blocked *.smilefrog.net *.google.com *.baidu.com *.bing.com;
    if ($invalid_referer) {
        rewrite ^/ https://www.smilefrog.net/wp-content/uploads/2018/08/smilefrog_no.png;
        #return 404;
    }
    expires      30d;
}