At the time, unfortunately, there was no particularly good way to upload photos to the service from a Mac. But now there are two good ways: Picasa Web Albums Uploaders for the Mac.
This small free ultra-cool package includes two pieces of software:
- For iPhoto users: Picasa Web Albums Exporter is an iPhoto extension. After you install it, you can use it from within iPhoto, by choosing File > Export (or maybe Share > Export in some versions of iPhoto). It's even easier to use than iPhoto's built-in exporter, and results (imo) in web albums that look nicer than the ones iPhoto creates. I've used it several times, and each time I was delighted with how smoothly it worked.
- If you don't use iPhoto: Picasa Web Albums Uploader is a standalone app that lets you drag and drop photos from the Finder; then you click the Upload button and it posts them to your web albums.
(Note that you have to sign up for a Picasa Web Albums account before you can use either of these tools. I'm not sure how long the waiting list is for accounts at the moment; if you want an account, go to their main page and sign up in the "Sign up" box on the right.)
I've been looking for a good way to post photos online for years; I really like this system. (I've looked at a number of popular approaches, but none of them have previously appealed to me.) If you want to see some samples of what the resulting albums look like, and you haven't already looked at the ones I've posted, drop by my public gallery.
(Yes, this is made by the company I work for. But I have no affiliation with this particular product other than as a very happy user. After using and enjoying a prerelease version of the software, I considered offering to write the Help for it, but one of my colleagues (a Mac expert who's written highly regarded Mac books, so much better qualified than I) beat me to it.)
P.S.: Here's a Google Blog post about this uploaders-for-Mac project; the blog entry was written by Ted Bonkenburg, the engineer who started the project. Turns out that he and Greg Robbins, another person who worked on this project, were part of the amazing Graphing Calculator story, and if you haven't read that story you should, even though it's entirely unrelated to the Picasa Web Albums Uploaders. (Where else can you read about two engineers breaking into Apple to secretly create incredibly cool software?)