How does Picasa handle edits?
When you edit a photo in Picasa, Picasa does not modify or replace the original photo, but rather records the edit in an “edit list” associated with that photo. Each time you perform another edit, that edit list is updated. If you close Picasa and come back another day, the edits are re-applied to the photo as it is displayed. This allows you to undo any edit and any time, even after closing and reopening. There is no need to “save changes”; Picasa does it automatically. When you finally print, upload, email, export, burn, or otherwise use that photo in Picasa, all those edits are applied once to the original to produce that output, but the original is not moved or modified in any way. This way there is no accumulations of JPG compression artifacts if a series of edits is applied over time.
