Images are a collection of a boot loader, operating system, and a root filesystem. There are public images provided by Google and various open source communities. We can use these images to create an instance. GCE also provides us capability to create our own image which we can use to boot instances.
It is important to understand the difference between an image and a snapshot. A snapshot is incremental but it is just a disk snapshot. Due to its incremental nature, it is better for creating backups. Images consist of more information such as a boot loader. Images are non-incremental in nature. However, it is possible to import images from a different cloud provider or datacenter to GCE.
Another reason we recommend snapshots for backup is that taking a snapshot does not require us to shut down the instance, whereas building an image would require us to shut down...
