During the Google Home Hub (now rebranded Google Nest Hub) setup process you're asked to choose between Google Photos, curated art and a regular clock face for display when the Hub is not in use. The problem is it's not easy to see how to later change this if you decide you'd prefer something else. Fortunately there's a really easy fix.
Launch the Google Home app and select the Home tab. Scroll down to and click on your smart display, then tap the Settings cog at the top right.
Toward the bottom of the screen you should see the option Photo Frame - tap this.
On the next page you can switch between Google Photos, Art gallery, Full screen clock and 'Experimental' simply by tapping on that option.
If you choose Google Photos you'll be given the chance to select only certain albums for display, while with Art gallery you get a choice of Featured photos curated from the web, Fine art, Earth and space, and Street art. Choose Full screen clock and you get six different options to choose from.
Below these options are toggles for also display the time and weather, and right at the bottom you can choose how often ambient images change.
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