I think the least complicated way to do this would be to use a simple image viewer app like viewnor or gThumb.
You can undecorate the window to make it seem more naturally like a photo frame. The advantage here is you can move it around and adjust the window size to suit your preferences just like any other window.
Different image viewers
have different features, so experiment.
I'd also suggest that you use a different app to your default image viewr so as not to interfere with normal image viewing when you need it.