You need to use the Google Photos app on a mobile device or Google photos in a browser logged into the same account as your TV to set it up.
In the Google Photos app
Create a new Album (or shared Album, we do this and we all add photos to the TV screensaver album) and add some pics, call it something obvious like TV Screensaver Album
Go to Library / Utilities / Manage photo frames. Select your Android TV device and select your newly created album.
Done!
On your TV try it out by running the Pics screensaver, it might show stock photos first.
Enjoy
