On ubuntu, Picasa doesn't allow you to download webalbums since firefox doesn't know what to do when you click on the download webalbum button in the picasa webalbum. There is however another way to download pictures in a webalbum when you use Firefox as your browser:
Unlike other photo sharing sites, Picasa Web Albums offers a way to download an entire album, but that requires Picasa 2.5 or later. If you don't want to install the application or you use Mac or Linux, there are other ways to download a photo album.
For Firefox, you can use DownThemAll, probably the best download manager add-on. After installing the extension and restarting the browser, 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/DownThemAll in the contextual menu, expand the Filters section and check "Images". Don't forget to enter the folder where you want to save the photos.
Another way is tio install Seamonkey. This cousin of Firefox does know how to handle the picasa webalbum links and nicely starts Picasa that allowes you to properly download any webalbum.
Tweaking Ubuntu >