The photobooth application was written in Python and I was able to get picasawebsync (https://github.com/leocrawford/picasawebsync) working. Even though the documentation says it's probably no longer working, I can verify that it was working for me as of September 2016 which is after the Picasa deprecation. I had to edit the source a little so that I could call it from my app instead of the command line.

Another challenge was that I couldn't find a good application to display the Google photos album. Nothing I found would display any new photos added to the album after the slideshow had begun while also displaying everything in a continuous loop. I ended up writing a second small Python app also using picasawebsync to periodically sync the photos to a second Raspberry Pi which was hooked up to a projector and display them looped in a random order.