I have added my wife to an account on AppleTV, but when I switch to her she is welcomed by name. However, we cannot access her photos which she has designated to be available to AppleTV. What have I failed to do?

Where are these photos? On a Mac, on a different device, or in iCloud?

To access photos that are on a Mac, the folder with images has to be set for Home Sharing. Then to access Home Sharing photos, use the Computers app on Apple TV, then the Photos tab.

Home Sharing requires a matching Apple ID account to be set for Home Sharing on both devices.

I’m afraid that you will have to remove the current account for Home Sharing first, before you can add a new one.

You may suggest to Apple to change this for future versions tvOS. Feedback - Apple TV - Apple.