I don't really know for sure about photo's, but the only way to get anything playing on the Apple TV if it's not MP4 format is by transcoding it. That is where those new DS214play or QNAP TS-X51 series come in, since those come with hardware transcoding (Intel Quicksync). Theoretically this means the Apple TV should be able to play anything you throw at it, but I've read in practice it still doesn't work quite as smooth as it is supposed to be. Plex is probably still the best option, since it'll also do transcoding, but unfortunately not (yet?) in hardware. This means you'll need a pretty powerful NAS for this to work without hiccups.