I discovered that if I press Pause during an Apple TV photo slideshow while a transition is in progress then it will stop midway through the transition. I've been having fun catching dissolves from one image to another. It often turns a family member into a ghost, haunting another scene. I'll add that to the list of tricks worth doing with my Apple TV, which includes watching photos go by in random order in screensaver mode, trying to name where each photo was taken. With thousands of photos from dozens of vacations, events, and other adventures, it's a challenge to name the location before the other family members can!