iPhoto uses nondestructive editing, which means that a photo’s quality never degrades, no matter how many times you edit and resave it.
Here’s how nondestructive editing works: When you import a photo, iPhoto stores the original version and never makes changes to it. After you open the photo and edit it for the first time, iPhoto saves the edited version separately, so you have two versions: the original photo and the edited photo. The original version is unaffected by your edits.
Every time you make more edits to that photo, iPhoto reopens the original version, applies your new edits, reapplies all the previous edits, and then saves this new edited version (again, it’s saved separately from the original). This means that the edited photo is never more than one version away from the original. This ensures the highest quality for an edited image.
If a photo was imported into an earlier version of iPhoto (iPhoto 6 and earlier), nondestructive editing doesn’t apply unless the photo was never edited.
For photos imported into earlier versions of iPhoto, and edited either in iPhoto or in another application, you can still use nondestructive editing by undoing all the edits to date. Do one of the following:
Revert to the original photo (select the photo and then choose Photos > “Revert to Original”).
Reimport the original photo into iPhoto.