It seems strange that there is nothing available that will easily do this for Mac users.
I did find this blog post which seems fairly easy. It does depend on you having Python installed though.
Extracted from blog post
The key component to the migration was a python script I found
developed by Nathan Van Gheem last year, which used both services
api’s to migrate not just pictures but albums and much of their
metadata between the services.
So here we go, these instructions are for mac users who are in the
lucky position of having python installed by default, but for windows
are pretty similar are you have installed Python.
From a terminal window, install the two photo service api’s and the
threadpool packages for Python
To use the python script you will need to authenticate against each of
the services to access your pictures, for picassaweb this is your
username and password, for Flickr because you are using in effect an
external application to access your pictures you will need a Flickr
API Key and secret.
From your Flickr account go the App Garden and apply for your own
non-commercial API key. You need to provide a few details, but will
end up with the api key string and a separate secret key string,
Now you have all you need..
Remember to make your downloaded script executable
chmod +x migrate-flickr-to-picasa-nokey.py
And run it
You should be asked to enter firstly your picassweb details, then your
flick api key and secret, you should then be taken to a flickr webpage
to confirm you want the script to have access to your pictures.
This script will move all the photos and sets from flickr over to picasa.
That will require getting authentication information from both services...
Authenticating with Picasa...
Picasa Username(complete email):firstname.lastname@example.org
Authenticating with Flickr..
Flickr API Key:e97df11b147f9c46c2ec002b29f479df
Flickr API Secret:8fe34c331abbce32
Press ENTER after you authorized this program
Confirm that it’s ok, go back to the terminal window and press enter
for the script to continue.
The process will take some time, so go grab a coffee and look forward
to seeing your pictures in picasweb !