Conference room mode custom background?

If you want to set a conference room mode custom background image…where is that image supposed to come from? I know it's supposed to be the linked iCloud account…but conference room Apple TVs don't have iCloud accounts. By definition, they're not owned by anybody.

I created a bogus iCloud account just for the purpose of syncing photos, but wow that was a pain—it's no fun thinking up answers to security questions for a corporate shared account—and it required me to create an additional account on my Mac just for this purpose.

How is one supposed to do this?