Google stopped supporting Picasa3 on March 15, 2016, and removed the installation file from the Picasa and Picasa Web Albums Help website. Read more about Picasa's "retirement".
The latest version for Windows computers is Picasa 3.9.141 Build 259
The latest version for Mac computers is Version 3.9
See below for directions how to find the latest Picasa installation file for Windows and Mac.
If you have already downloaded the Picasa installation file, save the file to several different places such as external drives, flash drives (USB), CDs, Google Drive. You would use that file whenever you need to install or reinstall Picasa3 to another computer, to install after your computer was reformatted, or for any other reason you need to reinstall.
For Windows users
A Product Expert has made the final Picasa3 setup file for Windows available at this THIS link (picasa39-setup.exe) or HERE.
You could also get the setup file, at your own risk, from Filehippo
For Mac users
At your own risk: https://picasa.en.uptodown.com/mac
We have been informed that this does no longer work on the latest Macs.
How to install the latest version
Check your version of Picasa by going to Help > About Picasa
In the "About Picasa" box under the Picasa logo is the version and build number
If your version is earlier than the final version 3.9.141 build 259, do the following:
Locate the latest version of Picasa .exe file (see section above) and run the installer over your previous version
You don't need to first uninstall your present Picasa version
The installation process will find the existing Picasa database and photos on your computer.
Your photos exist on your computer and will not be harmed or lost during an installation
If Picasa fails to install, then uninstall your present version, go to Tools > Options > General tab. Under Automatic Updates, change to "Don't check for updates" then click OK. Install the latest version of Picasa.
For Ultra HD-screens (Windows): right click on the Picasa-icon on desktop and choose Properties > Compatibility: you can switch-on an option for High DPI.
For new users