After you've taken your photo and reached the final screen of the Snapshot experience, you will see an option to download your photo:
Tap the 'download' arrow under your photo
Tap 'Download' in the pop-up window
Tap the blue arrow that appears in the search bar of Safari
Tap the magnifying glass next to your photo
Select all of the Snapshot photos you'd like to save to your camera roll
Tap 'Save Image'
