Using PhotoStream on the Mac for pics transfer
Once Photostream images are exported to the Mac, they can safely be removed from your iPhone camera roll or from the Photostream itself. The images on the computer are local copies and are no longer directly tied to the Photostream. The photos will remain on your computer until you delete them. 


  Since your Mac and PC have more storage than your iOS devices, you can choose to have all of your My Photo Stream photos automatically downloaded. In iPhoto or Aperture preferences on your Mac, be sure to select Photo Stream > My Photo Stream > Automatic Import. All of your photo stream photos will be imported into your Events, Projects, Photos, Faces, and Places folders in iPhoto or Aperture. On your PC with Photo Stream enabled in the Control Panel, all of your photos will be imported into C:\Users\\Pictures\Photo Stream\My Photo Stream. 


ᔥ Apple Knowledgebase: iCloud: Photo Stream FAQ