Works for me, although not so obvious at first how to start. Try this:
- select a photo album in Photos
- start slideshow
- when first pic comes up on iPad screen, tap the screen
- slideshow is interrupted, menubar appears
- tap on Airplay icon at top right, then select your Apple TV
- start the slideshow again - now it's displayed on your TV
Not sure if this is the quickest way, but that's how I got it to work