After a new image is built, it can be uploaded using the same command as before, with the --update-existing flag added:
openstack overcloud image upload --update-existing
Note that if the new image is a ramdisk, the Ironic nodes need to be re-configured to use it. This can be done by re-running:
openstack overcloud node configure --all-manageable
For TripleO release Mitaka, the boot configuration command is:
openstack baremetal configure boot
Now the new image should be fully ready for use by new deployments.