there is an easy solution for this. 1) create a google photos album with a single photo in it 2) in the "Google Cast" mobile app, go to Settings --> Backdrop,

there is an easy solution for this. 1) create a google photos album with a single photo in it 2) in the "Google Cast" mobile app, go to Settings --> Backdrop, and uncheck/turn OFF everything except Google Photos 3) select your "single photo" album 4) profit