Thanks for reaching out. To verify, are you casting your Google photos to the Chromecast or you're trying to set your Google Photos on the Ambient mode for the Chromecast? In the meantime, please check this support website article to learn more about the Ambient mode feature for your Chromecast with Google TV (4K).
I haven't heard from you so I’ll be locking this thread after 24 hours. If you're still experiencing problems, please feel free to start a new thread and we'll be happy to help.