If statements, string manipulation and file detection

[oops_im_a_sysadmin]

Hi, I'd like to use some if statements in a config. I know if in nginx is
evil, but I think it's what I want. Here is the pseudo-nginx-conf-code for
what I need: http://pastebin.com/BxrtZ695 — can someone please help me get
the syntax right? In english, the idea is: if the requested filename ends
with either "F.jpg" or "M.jpg", and another file with almost the same name
but the _other_ ending instead does not exist, break/fail.

Thanks for any help!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,241010,241010#msg-241010