One such app that's especially useful for iOS users, will send photos to a Chromecast. The app is called Photo Cast and is available for free.
After installing the app the only setup required is to grant the app permission to access your photos, and tapping on the Chromecast icon to select the device you'd like to send the photos to.
Then, any photos you select will show up on your TV automagically. You can even start playing a slideshow, with an adjustable timer for each slide.
Some downsides currently are the inability to send videos from your iDevice to Chromecast, and of course there's no way to continue a slideshow while the app runs in the background.
Download Photo Cast for Chromecast.