I learned something about Picasa today: when you edit a photo, it stores all the modifications as extra fields in the JPEG file and doesn't modify the displayed image. Until you export the image from Picasa, other tools only see the original image.
I have a bunch of photos from before 2006 that I salvaged from an old Thinkpad, and which I'd touched up with Picasa, and I want to upload them to Flickr. But because the touch-ups are all in metadata, none of the images reflect this.
I downloaded the latest version of Picasa for OS X, imported all of these photos, and amazingly it seems to have correctly applied the changes (which were made with a much older version of the tool). They're not pixel-for-pixel identical -- I presume that some of the enhancement algorithms have changed -- but they're damn close.