In August 2016, Google dropped support of Picasa Web Albums — picasaweb.google.com. Only links to photos and albums whose URLs use a user ID number (and not a username) are valid now. Fortunately, there is a way to download all the public photos even if you know Picasa username only.
In most cases, you can get Google user ID simply by requesting an RSS feed. All you need is to substitute a username in the URL
http://photos.googleapis.com/data/feed/api/user/username. The 21-digit number (for example, 123456789012345678901) in the title is the Google ID. Then you can access the list of albums by the URL
However, there were some Picasa accounts not linked to Google profile. RSS feeds for such accounts show a username instead of an ID. In that case you need to retrieve another URL:
https://picasaweb.google.com/data/feed/back_compat/user/username. In the source of the feed you need to find the string '<gphoto:rsslink>'. Immediately after each of such tags you will see the URL
This URL (
&kind=photomay be omitted) contains direct links to all full-sized photos in the album, with original names. You can easily get it by almost any bulk download utility.