How do I specify my own image in the terraform-google-bastion-host project?

I have successfully used the default settings for github terraform-google-bastion-host

Now I’m trying to set the image or image_family to use my own Packer’d image in a GCP Container Registry. What I’m missing is the syntax for setting the image or image_family variables. I’m trying the following, but it’s not working.

image_family = ""

BTW, thanks for maintaining great products like Terraform & Packer. With great projects to learn from and use.