OpenStack images for CentOS are available from cloud.centos.org, including current images for:
Follow the local setup instructions before starting.
Download image from the above URL, then run:
openstack image create --file ~/Downloads/CentOS-6-x86_64-GenericCloud.qcow2 --disk-format qcow2 --container-format bare "CentOS 6"
openstack image create --file ~/Downloads/CentOS-7-x86_64-GenericCloud.qcow2 --disk-format qcow2 --container-format bare "CentOS 7"