Picasa Web Help
Use Google Takeout.
Alternatively, if you're using Picasa Web Albums, you'll already have a copy of your photo albums on your personal computer, so there's not much of a concern with getting your data "out" of Picasa. However, if your computer is unavailable (e.g. it's crashed or stolen), you'll surely want to retrieve your photos from the web. One way to do this is to use the "Import from Picasa Web Albums" feature in Picasa:
You can also download albums from Picasa Web's web interface.
Note: If you want to download all of your videos from Picasa Web Albums, you need to use Google Takeout.
There are a number of different ways to get data into Picasa Web Albums. See the "Uploading to Picasa Web Albums" section on Picasa Web's Uploading page for details.
See the "Mac Uploading Tools" section on Picasa Web's Uploading page for details.