Smart displays with Google Assistant have a few different options for what appears when not in use. They can show a full-screen clock, images from Google Photos, or an art gallery. Google also included experimental support for showing images from Facebook or Flickr accounts, but this has now been removed.
A new Reddit thread by u/big-boss_97 pointed out that the options vanished. A verified Google support employee later commented that the removal was intended, and went on to say that there are no plans to bring the functionality back. "We typically make numerous changes to the Google Home app within each update," the comment said, "[and] as part of the 2.34 rollout we removed an experimental SDK associated with an ambient photo experience that included Facebook and other third-party photo apps. [...] We currently have no plans of returning this feature."
The options were previously available under the Photo frame options in the Google Home app, and were marked as 'experimental' the entire time they were available. The only viable workaround is to occasionally export your images from those services and upload them to a specific Google Photos album, which can then be selected in the Google Photos page.