Picasa 2.5 or later offer a way to download an entire web album but if you don’t want to install the software below are some ways to download Public or Private Web Photo Albums from Mac, Linux or Windows.
Using DownThemAll Firefox Plugin
Install DownThemAll Plugin and Restart Firefox, Now go to the album you want to download, click on the "RSS" link from the sidebar and you’ll get the album’s feed. Right-click on the page, select DownThemAll in the menu, expand the Filters section and check "Images". Make sure to enter the folder where you want to save the photos.
Using Gladinet Cloud Desktop application
Gladinet lets you mount the whole Google Picasa Web Albums into Windows as a virtual drive with each web album appearing as local virtual folders. Download web album is as simple as drag and drop folders into my another local drive. Read and Download from Gladinet Cloud Desktop Review – Mount any web storage.
Using iPhoto in Mac
Copy the URL of the RSS feed from your picasaweb album.
In iPhoto goto>File>Subscribe to Photo Feed<
Paste the URL and iPhoto will copy the photos to your mac.
Using Picasa Web Utility ( Google App Engine Powered )
Sign into Picasa Utility with your Google Account and authorize Picasa Web Utility to access your photos. Select any album that you want to download locally from the list. Under options, choose “Original Photo Size” and click Generate Codes. Save the text file with a .html extension and open this HTML file into any web browser and choose Save As, entire page. This will download all the Picasa images to your hard drive.
Using Picasa Webalbums Assistant
Allow you to download entire albums or parts of albums from Picasa Web albums. Download public albums buy entering the UserID of the owner. Download private albums by pasting an invitation URL. Includes thumb nailed previews of the pictures with check boxes to select only the pictures you want to download. Download here.
Picasa Tiny Downloader
A minimal java app to download a whole Picasa Web Album starting from the RSS link. Download here.