H/T to Brian Briggs for introducing me to this awesome idea!
Create an album in Google Photos specifically for the Chromecast
Open up the Google Home app that you used to set up your Chromecast. On the home screen tap on the Chromecast's icon and then click on the gear to access the device settings.
Scroll down the settings until you see Ambient Mode. Click on Ambient Mode.
Select Google Photos as your photo source.
Select the photo album that you created in step 3 as your source.
Upload photos to the photo album using Google Photos.
Use the Google Photos app on your phone or computer to manage the photo album. You can also share the album with anyone who you want to add photos.