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.

Source: Picasa Blog

So download and keep the installer somewhere safe!