At ExtraVM we provide mountable Netboot.xyz ISO which lets you network install any common Linux operating systems, liveCD images, and more. You can read about Netboot.xyz at the link below.
If you prefer to do a manual install of an operating system over VNC, we highly recommend using the Netboot.xyz installer.
If you require a different operating system, please contact us. We typically are able to mount ISO images at customer request.