Comment 2 for bug 1814633

The available image ids for the GCE regions is queried at bootstrap time and is sourced from here:

http://cloud-images.ubuntu.com/releases/streams/v1/com.ubuntu.cloud:released:gce.json

TL;DR; It's possible to copy and edit the metadata or generate your own and place it in a directory (you need the index file and the file above). You then bootstrap with --metadata-source=<dir>. More info in this post https://discourse.jujucharms.com/t/juju-deploy-series-centos/1294/3

If you did go and create your own images, I do not know if said images would still be available from the base path "projects/ubuntu-os-cloud/global/images/"? I suspect we'd also need to introduce a model config item to allow the base image path to be specified.