iOS 9’s Photos app classifies your photos into smart groupings known as Moments, Collections, and Years. Furthermore, the Photos app will automatically create and display thumbnails for all your photos.
If you would like to prevent other people from peeking into your private photos, there's a simple trick to hide photo thumbnails from the smart groupings. This guide will teach you how to hide photos from the Photos app.
Open the Photos app > Photos tab and select the items you want to hide. Tap on the Share button displayed at the bottom left corner of your screen and then tap on Hide:
iOS 9 will display a warning message that reads, "This photo will be hidden from Moments, Collections and Years but still visible in Albums." Tap on the Hide Photo(s) button to proceed. To view or restore hidden photos, you simply have to go to Albums > Hidden. Select the desired photo(s) and then tap on Share > Unhide.