[Iceman90]

The Chromecast is an interface to another device. For this to work, you'd need to keep a tab open on your PC with the slideshow on it at all times - probably not what you want. Roku would be very similar unless you create a custom Roku app, or find an existing app to do this.

My suggestion would be to get a cheap Android TV box which has onboard storage ($60 or less). Then create an album in Google Photos, and set up the Android TV box to show that album as a slideshow. Then when you are ready, you can dump more photos into that album on your PC, and it should be updated on the TV next time the album loops.