Chromecast with Google TV is similar to Amazon Fire TV. "Chromecast with Google TV" is quite a mouthful, so I'll just call it Google TV. Surprisingly, you can order Google TV on Amazon. It has a remote and a dongle that plugs into the back of your TV. The remote has special buttons to let you open YouTube, Netflix or do a voice search. You can reach Google Chromecast support at 844.400.2278.
If you Google TV is inactive for 10 minutes, a screensaver will start in what Google calls Ambient Mode and you will see a photo with a thumbnail overlaid on it., If you select it with your remote, a series of photos will appear as a slideshow. At the time I am writing this, you cannot change the time after which the the slideshow appears. You can set the Google TV to never turn itself off, so once the slideshow starts, it will keep running.
Setting up the Google TV screensaver can be a pain in the rear; at least it was for me. Google support agents were clueless and I ended up teaching them about it. As you follow the steps below, please be aware that some things may have changed. Also, perhaps Google support agents will have improved their knowledge by the time you call them. You can reach Google Chromecast support at 844.400.2278.
Again, please note that Google refers to its screensaver as Ambient Mode -- for reasons only known to the clowns at Google.
To setup your Google TV Screensaver, you will need both the TV and your smartphone.
From your Google TV
On the home screen, at top right, go to your user icon at top right and press the down navigation button.
Select Ambient mode.
Select Google Photos.
On your Smartphone:
Open the Google Home app
Tap your TV (You may need to tap the appropriate room first.)
At top right, tap the Setting icon that looks like a gear.
Tap Google photos once or twice to access the various albums available.
Tap the album you want to use as the source for your screensaver.
Back on your Google TV
Access the Ambient mode (as described earlier)
Select Personal photo data and show or hide the owner's name and album name.
Select Google Photos and choose whether photos will show in portrait orientation or show in pairs.
Select Personal photo curation and choose which albums are included,
Select Slideshow speed and choose how long a photo stays on the screen.