Architecture Simplified: GCP - Compute Engine

Images

While creating a Compute Engine instance on Google Cloud Platform (GCP), you can choose Image.

An Image contains boot loader, operating system, file system structure and any software & configurations which you want to be available after boot load.

A persistent boot disk is created for the instance when you use an image. The default size of boot disk as per the image you selected. But you can modify the size of persistent boot disk, if needed.

These are preconfigured Images
You can choose Windows and Linux Images.
These are provided and maintained by GCP, open source communities or third-party vendors.
There is an additional cost involved for third-party vendor images.
By default, all projects have access to these public images.
Many Windows and Linux Images are available as public images with its pre-defined specifications.
Most public images are available at no additional cost.
Premium public images have additional cost involved.

You can create an image from your existing VM instances so that next time when you need similar boot configurations, you can use the same image.
These are created from boot disks and other images.
Since these images are created by you, so you would have control access on it.
These are available only to your project.
You can use one custom image and make some more changes to it. Then you can create another image which will have your updates as well. Next time, when you need more VM instances with the same configurations, you can use the same.
If you are making changes to your custom images regularly, then you can group them as image family.
The image family contains all the custom images. But it points to the most recent image only. So that if you want to create more VM instances using these templates, you can simply use the image family which is pointing to the latest image. This way you will not have to worry about updating the reference of latest custom image.
If you want to use previous version of image in the image family, then you need to deprecate the latest version as image family never points to older images.

These are verified public images with additional security features such as secure boot, vTPM (virtual Trust Platform Module), measured boot, integrity monitoring.