Desktop application
As of March 15, 2016, we will no longer be supporting the Picasa desktop application. For those who have already downloaded this—or choose to do so before this date—it will continue to work as it does today, but we will not be developing it further, and there will be no future updates. If you choose to switch to Google Photos, you can continue to upload photos and videos using the desktop uploader at photos.google.com/apps.

Downloading Picasa has been disabled. Maybe... ? http://download.cnet.com/Picasa/3000-2193_4-10160334.html and other sites. Not sure if it can be activated, but it hasn't just vanished from the planet.