From the iPhoto Library
Picasa for Mac has been designed to play nicely with iPhoto.
Picasa doesn't store any of your photos. It simply scans your hard drive and displays the photo files it finds. In this way, Picasa folders represent the actual directories containing photos on your computer's hard drive.
Picasa handles the iPhoto Library in the same way, with a small variation made for the way in which the iPhoto Library organizes your photos: If you've made edits in iPhoto, each album or event in your iPhoto Library contains an 'Originals' folder and a 'Modified' folder. Picasa scans the photos and videos in both the 'Originals' and 'Modified' folders, and displays a single collective folder with the most recent version of each file.
The photos in your iPhoto Library will be displayed in Picasa in the 'iPhoto Library' collection as read-only files. The read-only status is necessary to ensure that Picasa and iPhoto work well together. You can do a lot with read-only files in Picasa -- upload, create a collage, create a movie, email, print -- but you won't be able to make any material changes: this includes editing, moving, deleting, adding tags and using Sync to Web.
Of course there's a solution: When you attempt to use Picasa to edit an iPhoto Library picture, Picasa will ask your permission to copy the entire folder containing that photo to a location outside of the iPhoto Library -- they're placed in a different folder under 'Pictures' titled 'Imported from iPhoto.' You'll then have access to the full range of tools that Picasa offers. It's also possible to import your entire iPhoto Library in this manner. Just go to the File menu > Import from iPhoto... Please be aware that this action will make copies of your entire iPhoto Library.