https://github.com/EagleWu/nginx-lua-p_w_picpathmagick
http://sihuo.com/cdn_assets/photo/201411/06/545ae4109ac1b891759542.jpg
http://sihuo.com/cdn_assets/photo/201411/06/545ae4109ac1b891759542_320x320.jpg
http://sihuo.com/cdn_assets/photo/201411/06/545ae4109ac1b891759542_640_640.jpg
set $p_w_picpath_root "/opt/wwwroot/";图片输出目录
set $convert_bin "/opt/app/ImageMagick/bin/convert"; 是p_w_picpathk安装路径
输出到 p_w_picpath_root 目录里面去 和原始文件在一起