[Disclaimer: This article is a copy-paste from here: http://www.labnol.org/software/download-picasa-web-albums/8573/]
Download your Picasa Web Albums without Picasa Software
If you are using Picasa Web Albums to store your photographs but don’t have the Picasa desktop software to download those images back to the hard drive, here’s a simple trick that will help you download original (full size) pictures from any of your Picasa Albums without requiring the Picasa application.
Step 1: Open Picasa Utility and sign-in with your Google Account* – use the same credentials that you use for logging into your Picasa account.
*Picasa Web Utility is a Google App Engine powered application so your Google password won’t be shared with the developer.
Step 2: Once you have authorized Picasa Web Utility to access your photos, it will show a list of all your existing Picasa Album. Select any album that you want to download locally.
Step 3: Under options, choose “Original Photo Size” and click Generate Codes. Picasa Web Utility will now create direct download links to all photos in that Picasa Web Album.
Open notepad, copy-paste those Picasa links here and save the text file with a .html extension. Now open this HTML file into any web browser and choose Save As -> entire page. This will download all the Picassa images to your hard drive.
Picasa Web Utility has other applications as well. For instance, you may generate HTML codes for thumbnails and create an instant photo gallery for publishing on a blog.
Related: Copy Flickr Pictures to Picasa Albums (and vice-versa)