server {
    # Internal image resizing server.
    server_name localhost;
    listen 127.0.0.1:9001;
    access_log off;
    image_filter_buffer 16M;
    image_filter_jpeg_quality 85;
    image_filter_transparency off;
    image_filter_interlace on;
    resolver 8.8.8.8 ipv6=off; # Using Google DNS

    set $width "-";
    set $height "-";
    if ($arg_w != "") { set $width $arg_w; }
    if ($arg_h != "") { set $height $arg_h; }

    location /safe_image {
        if ($arg_url = "") { return 404; }
        set_unescape_uri $arg_url;

        if ($arg_crop = 1) {
            rewrite /safe_image?(.*) /safe_image/crop?$1 last;
        }

        image_filter resize $width $height;
        proxy_set_header Accept-Encoding "identity";
        proxy_set_header Referer "";
        proxy_pass $arg_url;

        location /safe_image/crop {
            internal;
            image_filter crop $width $height;
            proxy_set_header Accept-Encoding "identity";
            proxy_set_header Referer "";
            proxy_pass $arg_url;
        }
    }

    location /wp-content/uploads {
        if ($arg_crop = 1) {
            rewrite /wp-content/uploads/(.*) /wp-content/uploads/crop/$1 last;
        }

        image_filter resize $width $height;
        alias /var/www/example.com/public_html/wp-content/uploads;

        location /wp-content/uploads/crop {
            internal;
            image_filter crop $width $height;
            alias /var/www/example.com/public_html/wp-content/uploads;
        }
    }
}

proxy_cache_path /var/cache/nginx/wp_image_cache levels=1:2 keys_zone=image_cache:20m inactive=30d max_size=10g;

server {
    listen       80;
    server_name  example.com;
    root         /var/www/example.com/public_html;

    # ...

    location /wp-content/uploads {
        if ($arg_w != "") { return 418; }
        if ($arg_h != "") { return 418; }
        error_page 418 = @image_svr;
        try_files $uri =404;
    }

    location /safe_image {
        if ($arg_url = "") { return 404; }
        error_page 418 = @image_svr;
        return 418;
    }

    location @image_svr {
        expires 1M;
        secure_link $arg_d;
        secure_link_md5 "$uri$arg_w$arg_h$arg_url your_key";
        if ($secure_link = "") { return 403; }
        if ($secure_link = "0") { return 410; }

        proxy_pass http://127.0.0.1:9001;
        proxy_hide_header X-Frame-Options;
        proxy_cache wp_image_cache;
        proxy_cache_key "$proxy_host$uri$is_args$args";
        proxy_cache_valid 30d;
        proxy_cache_valid any 10s;
        proxy_cache_lock on;
        proxy_cache_use_stale error invalid_header timeout updating;
        proxy_http_version 1.1;
    }

    # ...
}