I've seen this happen when the img failed to download for any reason and
its size is 0. Check "/var/lib/tftpboot//boot/CentOS-6.4-x86_64-vmlinuz"
and make sure is ok, also check "/var/lib/tftpboot//boot/
CentOS-6.4-x86_64-vmlinuz"
hope that helps.
Light travels faster than sound. This is why some people appear bright
until you hear them speak