If your media is on your Mac and in iTunes it allows you to access all of it - music, videos, photos. You can use your pictures as a screensaver. There is no need for an Apple Music subscription.
But an even better solution is if you have a Plex server on your Mac and you have an Apple TV 4. The plex app will handle everything in your iTunes library, as well as things that iTunes doesn't handle, like blu-ray disks. It is much easier to use. However it doesn't handle slideshows at the present, and won't do a screen saver.
AppleTV 4 also has infuse, which reportedly does a better job with high resolution audio, such as DTA-MA on blu-ray disks. Not as easy to use as Plex.