I recently got a Google Chromecast, and was looking for a way to push custom images as a screensaver for it.
Though there's no obvious "set it and forget it" that runs headless, you can easily run the built-in Chromecast wallpapers in FullScreen mode.
Just click to that page, use the Chromecast extension, and type F11 to put it in full screen mode.
If you discover any Chromecast Screensaver apps, please add them in the comments!