OK success! I upped the loglevel to "debug" in httpd.conf. When trying
to access images the following messages appeared in error_log:

(38)Function not implemented: core_output_filter: writing data to the
network

A google search later and I found the following explanation of this
error:

"Apache uses the sendfile syscall on platforms where it is available in
order to speed sending of responses. Unfortunately, on some
systems, Apache will detect the presence of sendfile at compile-time,
even when it does not work properly. This happens most frequently when
using network or other non-standard file-system.
Symptoms of this problem include the above message in the error log
and zero-length responses to non-zero-sized files. The problem
generally occurs only for static files, since dynamic content usually
does not make use of sendfile.
To fix this problem, simply use the EnableSendfile directive to disable
sendfile for all or part of your server. Also see the EnableMMAP, which
can help with similar problems."

After setting "EnableSendfile off" in httpd.conf and restarting httpd
it works as it should.