iCloud photostreams are great for sharing photos with people. Sometimes you want to save some of the photos in the shared photostream so that they remain on your device when the photo stream is deleted, or if you unsubscribe to save space on your device. To save photos, follow the process outlined below. This is specifically for an iPhone on iOS 7. It should be similar on an iPad, although some of the button locations may be different.
Select the photostream that contains the photos that you want to save to your device
Tap the 'Select' button in the top right of the screen. Now tap on the photos that you want to save. A blue tick icon should appear on the photos that you have selected.
When you have selected all the photos that you would like to save, tap the share button at the lower left of the screen (the box with the arrow pointing upwards).
Tap the 'Save to Camera Roll' button.
That's it. You now have a copy of the photos stored on your device that will still be there even if the shared photo stream is removed by the person who shared it.