I don't know how digital picture frame sales are going these days. A few years ago, they were all the rage, but I would think that the ability to carry and display your favorite photos on a portable device may have dulled desires for them.
Of course, the stunning Retina displays on iPads and iPhones make a great place to show your pictures, and the little free universal app Picmatic does a nice job of displaying your favorite photos, especially on an iPad.
You download the app, point it at an album, choose some music if you like and the app will display your pictures in a variety of frames that do animated flips as they change images. You get some choices about your layout, the randomness of images and whether the photos have frames around them or touch each other. The app displays the current time if you want it, and music selections can be changed with gestures by swiping on the screen. The only thing I don't like is the music track name stays along the bottom, as well as the button for activating the settings. I think it should disappear after a few seconds and not distract from the photos.
There is a Pro version for US$1.99 that gives you some additional layouts, but I found the free ones fine for my needs. There are plenty of similar picture frame apps, but Picmatic works well, and the price is just fine.
Picmatic is a good use for an idle iPad or iPhone, and it makes a nice conversation piece. The app requires iOS 6 and is optimized for the iPhone 5.