Images contain an operating system which can be cloned to a volume and used to boot a cloud instance. These are different to just a base installation of an operating system, as most of the images we provide include tooling called cloud-init which retrieves configuration for both our cloud, and about the specific instance that you are creating.
Having a wide range of images available will hopefully allow you to find one that meets your requirments. If they don’t you are always able to upload your own images to use within your project(s). See Uploading images to Catalyst Cloud.
You are able to take a ‘snapshot’ of an instance once you have customised it and use that to start additional instances. This can simplify building a solution (or complicate it!). See Using Snapshots.
Once you have created a new instance from an image it is always a good idea to perform an upgrade of the packages installed, and to continue to do this in an on-going basis. It is part of the shared responsibility mode that you are responsible for the security of the software within your instance(s).
We update on a regular basis the images we provide, when they are still supported by the providers. However these updates won’t flow into your instances unless you rebuild them using a new image as noted just above. As part of this process we test them to ensure they are operating in a way that we expect.