You might need to use the command similar to one below if you are on redhat distibution like CentOS or Fedora...

...
mount -t nfs -o context=system_u:object_r:httpd_sys_content_t 192.168.1.7:/var/www/html/images /var/www/html/images

If that still does not work, then let SELinux allow Apache to read on the mounted directory..

setsebool -P httpd_use_nfs on
OR
sudo /usr/sbin/setsebool -P httpd_use_nfs on

References:
http://wiki.centos.org/TipsAndTricks/Selinux...
http://www.redhat.com/docs/manuals/enterprise/RHEL-5...
http://www.linuxconfig.org/HowTo_configure_NFS