My Linode has "unknown image" in the new cloud interface. What does that mean? How can it be unknown? All my sites are running just fine. It certainly uses one of the disk images that I have. But nowhere does it say which one of them is currently being used.
The "Unknown Image" text just indicates that the image used when deploying/rebuilding the Linode is no longer available on our end. If you deploy a new Linode based on one of our available images, the name of that image will be listed (Ex: "Ubuntu 18.04 LTS" or "Debian 9"). To be clear, this isn't an indication of a problem and your Linode will still function normally.
As far as knowing which disk is in use by your Linode, you can review your Configuration Profile settings under Settings > Advanced Configuration. If you have multiple Configuration Profiles, there's currently no way to tell which one was used during your last boot. To do that, you'll need to log into the classic Linode Manager. For your reference, Configuration Profiles allow you to configure boot settings, the kernel, and device assignments. When you issue a boot job on a Linode with multiple Configuration Profiles, a prompt will display that allows you to select which one to use.