Some more interesting data: sometimes it HIT cache, sometimes not..... depending of referer???? I do not know how... I cannot imagine why.

(consecutive log lines filtering by uri.... I did not remove any file or cache, even not restarted nginx at all)

107.167.108.187 - - [19/Nov/2015:05:22:22 +0100] "GET /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c HTTP/1.1" 200 2082 "http://xxxxxx.com/2014/09/significado-del-nombre-daniela/" "Opera/9.80 (X11; Linux zvav; U; es) Presto/2.12.423 Version/12.16" Exec: "0.020" Conn: "6" Mobile: "-" Cache: MISS (0.020 sec) - Upstream: "200"
107.167.108.187 - - [19/Nov/2015:05:22:23 +0100] "GET /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c HTTP/1.1" 200 2082 "http://xxxxxx.com/2014/09/significado-del-nombre-daniela/" "Opera/9.80 (X11; Linux zvav; U; es) Presto/2.12.423 Version/12.16" Exec: "0.000" Conn: "4" Mobile: "-" Cache: HIT (- sec) - Upstream: "-"
107.167.108.195 - - [19/Nov/2015:05:23:11 +0100] "GET /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c HTTP/1.1" 200 2082 "http://xxxxxx.com/2014/09/significado-del-nombre-daniela/" "Opera/9.80 (X11; Linux zvav; U; es) Presto/2.12.423 Version/12.16" Exec: "0.037" Conn: "8" Mobile: "-" Cache: HIT (- sec) - Upstream: "-"
37.132.144.110 - - [19/Nov/2015:05:24:28 +0100] "GET /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c HTTP/1.1" 200 2082 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:42.0) Gecko/20100101 Firefox/42.0" Exec: "0.024" Conn: "1" Mobile: "-" Cache: MISS (0.023 sec) - Upstream: "200"
37.132.144.110 - - [19/Nov/2015:05:24:31 +0100] "GET /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c HTTP/1.1" 304 0 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:42.0) Gecko/20100101 Firefox/42.0" Exec: "0.000" Conn: "2" Mobile: "-" Cache: HIT (- sec) - Upstream: "-"
107.167.108.195 - - [19/Nov/2015:05:24:45 +0100] "GET /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c HTTP/1.1" 200 2082 "http://xxxxxx.com/2014/09/significado-del-nombre-daniela/" "Opera/9.80 (Android; Opera Mini/11.0.1912/37.7126; U; es) Presto/2.12.423 Version/12.16" Exec: "0.038" Conn: "6" Mobile: "-" Cache: MISS (0.038 sec) - Upstream: "200"

and the files:

root@hyperserver [/var/nginx.cache/xxxx]# find -type f -exec grep -a "KEY: /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c" {} \;
KEY: /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c
KEY: /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c
KEY: /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c
root@hyperserver [/var/nginx.cache/xxxx]# find -type f -exec grep -la "KEY: /wp-content/themes/sahifa/timthumb.php?src=/wp-content/uploads/2014/06/Clotilde.jpg&h=50&w=50&a=c" {} \;
./7/d2/39815042547bf336bfe0c8de84e17d27
./6/73/c1463df2aaa737d2616ba5016c64f736
./2/9c/edda94a8edf68a32eb83011e918a59c2

As you see, multiple files for the same KEY (request_uri)... sometimes MISS, sometimes HIT... I cannot imagine why.

Please, could someone help me or give a clue?