As you may know, Apple macOS has discontinued support for 32-bit apps and Carbon (as of Catalina and later, from 2019). Picasa for Mac was never updated after this, so it doesn't work at all anymore on newer Macs.
If you want to use Picasa occasionally on a Mac, it turns out the best (and cheapest) way these days is to use the Windows build and run it under wine. (If you have Apple Silicon, you should make sure to have Rosetta2 installed for this.)
- First, you should install https://brew.sh/ (homebrew).
- After homebrew is installed, do this from a terminal window:
> brew tap gcenx/wine && brew install wine-crossover
- Then download a Windows Picasa build here: http://www.oldversion.com/windows/picasa-3-9-10-4
Now, you can run the installer (just double-click it) and it should make a Picasa3.exe.
- Find the Picasa3.exe and drag it to your dock (in the "documents" part next to the trashcan, because to the Mac it isn't an app). Clicking it will then launch wine and let you run Picasa.
- Adjust Tools->Folder Manager to index things on your Mac. For me, the Z: drive is my Mac volume, so I've added Z:\Users\[me]\Pictures and the rest to the list of watched folders.
Expected issues: you'll have to ignore most Google services, there's no retina support, and raw support is ancient (so you may want to turn it off). Similarly, integration with Finder is not very good. But it does work, and it's mostly still pretty fast (though it draws to the screen a little bit slowly).
Thanks to the Wine team, this works better than the old actual Mac build, which is something, but thanks to them anyway!