You can hide images from the Camera Roll in iOS’ native Photos app to prevent them from appearing in your image list, but hidden images are quickly discovered by prying eyes when someone holding your iPhone opens the “Hidden” album in the Photos app.
If you wish Apple took better care of your Hidden images, then you might enjoy a new free jailbreak tweak called HiddenAlbumLock by iOS developer smokin1337. Just as the name implies, this tweak locks the Hidden album and requires authentication to unlock it for viewing.