Having never used an Apple TV, I'm not clear on the first part of your answer.
Here is what we're trying to do: We operate a kiosk inside a golf equipment store. Golfers can book tee times at local courses at our kiosk. We want the TV next to our kiosk to be showing a slide show of golf photos and lists of specials at the courses (which could be saved as jpg's), while at the same time we use the iMac to actually book the tee times. Ideally, we want to come in in the morning, start the slideshow, and go back to our normal business without having to fiddle with the slideshow again. Maybe once or twice a week we'll change the photos.
The screen saver idea on the Apple TV sounds like it would be perfect as long as using the computer doesn't cause the screen saver to stop.