First, what gives? Second, is there any way presently to show my Google Photos collection on the backdrop?
You just need to create "Google Photos" folders. Just select some photos and add them to a new folder at photos.google.com, and the new folder will appear in your Chromecast backdrop folder list, where you can select it for display.
Note: if you want to add a bunch of photos at once to a folder (either by date, or by selecting a face using the facial recognition search,) you can select a single photo's checkbox with a click, then shift click another photo. All of the pictures in between will also be selected.