I set up an old iPad 4 for my grandma as a picture frame. I did as the poster above suggested, but with one modification - I selected an iCloud album so I could add new pictures to the rotation without physically going to her house (as that would be quite the commute). It's actually pretty cool because she sometimes calls and asks what the new photos are about - makes a nice conversation starter!
(Note: Having your iPad run 24/7/365 WILL shorten the life of the display - though this was a used iPad 4 and it's been running for just over a year with no issues yet! It was cheap and I only intended it to run for the holidays so it's already gotten more life than intended!)