Yup, I've been using that - but it's not really a slideshow. Each picture is visible for 20+ seconds, and there's no music. I haven't tested if you play music that the backdrop is still visible though.

Google+ and Chromecast are like 85% of the way there. I just want to open up the Google+ app on my phone, go to an album and "cast slideshow" and have that be a great experience. Right now it's horrible.