Hi,
Looks like I'm having a problem similar to yours.
I'm waiting answer from customer, if the problem was solved.
Searching about in internet, seems the problem is due to use nginx as a proxy, in scheme: nginx <-> varnish <-> apache
Perhaps, are you using nginx & varnish & apache scheme.?
If so, the static pages are being served by nginx, to speed up loading pages, and not by apache.
So, you can try to exclude images extensions from you nginx configuration, to make apache serve it.
Depending on the template you are using, you can simply remove extensions from the location directive.
Regards,
Netino