There are quite a few great articles on the Internet about how to make your Raspberry Pi into a a digital picture frame from a hardware standpoint, but I haven't found a good set of end-to-end instructions for the software side. I decided to go down that journey myself. I didn't want to have to keep a copy of my images on a USB drive (or the Pi's SD card). I just wanted the frame to pull images from our desktop where the wife has already spent time organizing and touching up the photos.
At first I had the Pi doing all the image processing (resizing to make things fit on the display plus a simple web interface) but found that it just couldn't keep up with processing and displaying a new image (from a large-format JPEG) every ten seconds. The images were frequently appearing corrupted. So instead I wrote some Python code that runs on my Windows desktop to stage the images for the Pi. Then all it has to do is grab them and display them.
You can find all of the code and configurations I used along with instructions here:
It's extremely easy to modify this code for your own desires. If people actually start to become interested in that work I'll put together a proper
HOWTO with more details and options.