SourceImage: The source image used to create this disk. If the source image is deleted, this field will not be set.
To create a disk with one of the public operating system images, specify the image by its family name. For example, specify family/debian-8 to use the latest Debian 8 image:
Alternatively, use a specific version of a public operating system image:
To create a disk with a private image that you created, specify the image name in the following format:
You can also specify a private image by its image family, which returns the latest version of the image in that family. Replace the image name with family/family-name: