Hide any photos you do not want others to see
Imagine you have just come home from a fabulous vacation and you are showing your grandma all the wonderful pictures you took. Then, suddenly, a certain photo comes up that you never meant to show her, but it is too late now. The good news is that if you have an iPhone, you can make it so those mistakes never happen.
In cases like this, it is better to be safe rather than sorry. The Photos app has a “Hide Photo” option that moves them to a special folder for your eyes only. The downside is that the folder itself is not locked, which means anyone snoopy enough to access your phone will be able to go through it.