Google provides two products for getting your photos into Picasa Web Albums:
Picasa Web Albums Exporter for iPhoto, an iPhoto plug-in that lets you upload your photos directly from iPhoto. If you use iPhoto for managing your photos, this is all you'll need to get your photos to Picasa Web Albums.
Picasa Web Albums Uploader, a separate application for uploading photos. If you don't use iPhoto, or you prefer to use a separate program for uploading, this one is for you.
What is Picasa Web Albums Exporter for iPhoto?
Picasa Web Albums Exporter lives right inside iPhoto, in the File menu (iPhoto 6 and iPhoto 4) or the Share menu (iPhoto 5). Exporter is a plug-in, rather than a separate application. Using Exporter, you can
Fill your Picasa Web Albums with photos.
Create new albums and upload photos to them.
See how much storage you have available for web albums.
What is Picasa Web Albums Uploader?
Picasa Web Albums Uploader is a separate application for uploading your
photos. Because Uploader is a full application and has room to stretch out,
it provides some features that Exporter doesn't have:
Add photos (or folders of photos) that aren't in iPhoto.
Picasa Web Albums for The Mac
Upload photos to Picasa Web Albums using iPhoto 6
From the Google FAQ:
What is Picasa Web Albums Exporter for iPhoto?
Picasa Web Albums Exporter lives right inside iPhoto, in the File menu (iPhoto 6 and iPhoto 4) or the Share menu (iPhoto 5). Exporter is a plug-in, rather than a separate application. Using Exporter, you can
What is Picasa Web Albums Uploader?
Picasa Web Albums Uploader is a separate application for uploading your
photos. Because Uploader is a full application and has room to stretch out,
it provides some features that Exporter doesn't have: