Does iPhoto copy all photos from Photo Stream, or just make references?
Unlike your iOS device(s) that have a smaller more finite amount of memory, your desktop/notebook systems are more than capable of storing the photos, as they're found in your Photo Stream, they're also automatically imported to your iPhoto Library (same source as the question above):


  But since your Mac has more storage than your iOS device, it
  automatically imports every picture from your Photo Stream into your
  photo library so you can edit, delete, and share the ones you want.


Take note, however, that there is a difference between your Photo Stream collection and your iPhoto Library. Attempting to interact with any photo being viewed while Photo Stream is selected will prompt you to view the copy in your Library.