from
http://picasa.google.com/support/bin...12569&topic=-1 Is Picasa available for the Mac?
Picasa isn't currently available for Mac OS. Right now we're putting all of our energy into making Picasa the best program we can. We realize that a lot of digital photographers would like us to offer a Mac version, and we may consider this option in the future.
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probably the main obstacle to Picasa development on OSX at present is iPhoto.