I was looking for a digital picture frame for ages, but the bigger ones are too expensive and an iMac on the wall doesn't cut it either.

Use Apple TV as a Digital Picture Frame

"This specially designed software allows you to use AppleTV as a true digital picture frame without any limitations ! Set the display duration for your photos anywhere from several seconds to days !
A standard AppleTV device is limited to a maximum delay of 20 seconds between two photos in the slideshow mode. We overcame this problem and allow you to conveniently set up your own delay between photos."

I am looking forward to their download solution - I contacted them and they told me there will also be a free trial version soon.