This is a discussion area for Harvester HCI.
You can upload custom images using an external downloadable URL e.g,
https://cloud.centos.org/centos/7/images/CentOS-7-x86_64-GenericCloud.qcow2.xz
https://cloud.centos.org/centos/8/x86_64/images/CentOS-8-GenericCloud-8.1.1911-20200113.3.x86_64.qcow2