The script is written in Ruby, and uses RubyGems. You will need to install the appropriate packages for your distribution. For example, in Debian and Ubuntu:
sudo apt-get install ruby rubygems libopenssl-ruby
The script uses Google's gdata RubyGem, so you need to install it:
sudo gem install gdata
Download and unpack the script from my GitHub gist. Create an account.yml file in the same folder, using the following template:
password: "my secret password"
Run the script:
Find your photos, nicely grouped by album, in the picasa_username folder.
I used Google's gdata RubyGem, and the Picasa Web Albums protocol guide and protocol reference.
The code is pretty straight-forward. There is one subtlety: for photos bigger than 1600x1200, the Web Albums API returns the URL for 1600x1200 thumbnails by default. To get the original uploaded file, I use the imgmax=d parameter in album feed URIs.
Motivation and Conclusion
I wrote this script because the Picasa desktop software does not seem to be able to download from Picasa Web Albums on Linux.
I hope you found my post useful. I welcome your comments, as well as your pull requests on GitHub.