# uses x_forwarded_for header as key
 limit_conn_zone $http_x_forwarded_for zone=conn_limit_per_x_forwarded_addr:10m;

            server{
                   proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

                 location /img {
                     proxy_pass http://backend;
                     limit_conn conn_limit_per_x_forwarded_addr 1;
                        }
                  #...
            }