It's old, but it's free and it still works: Pycasa. It's what I use to occasionally download Picasa albums. http://www.pycasa-download.com.ar/

I wrote a script once to download all images, dunno if it still works...

#!/usr/bin/python
# -*- coding: utf-8 -*-
import sys
import urllib2
from os.path import join
from lxml import etree

argv_len = len(sys.argv)
if argv_len not in (2, 3):
    print 'link to albums\' RSS feed is missing'
    exit(1)
if argv_len == 3:
    dstdir = sys.argv[-1]
else:
    dstdir = '.'

album_url = sys.argv[1]
if album_url.startswith('/'):
    album_url = "file://%s" % album_url

rss_data = urllib2.urlopen(album_url).read()
exml = etree.fromstring(rss_data)

urls = exml.xpath('.//item/media:group/media:content/@url', namespaces={'media': 'http://search.yahoo.com/mrss/'})
for url in urls:
    fn = url.rsplit('/', 1)[-1]
    fp = open(join(dstdir, fn), 'w')
    fp.write(urllib2.urlopen(url).read())

I did it with digikam. Everything was ok, but I couldn't download all the albums at once, as I wanted.

It's not actually fully true, View image from context menu gives a link with a component looking like s800 which you can replace with s1600 if you want. Or you can save it just as it is if it's of already acceptable quality.

Ah, sorry, misread your post re 'all the photos'. Feel free to discard both comments. However, there's RSS feed which links to all the photos and which is actually used by desktop Picasa...

Using the rss stream might be a simple solution too. I didn't test it.