In your "username" directory there is a "pictures" folder which contains "iPhoto Library". If you right-click on "iPhoto Library" and select "Show Package Contents" you will see two folders "Originals" and "Modified". Your unaltered pictures are in the Originals folder organized by date. If you have edited any of the photos, they will be in the Modified folder also organized by date.
Do not move files around at this level or your will mess up the iPhoto database. It is best to export any pictures you want from within iPhoto if you need them.
-howard