Oh you can hid images in the gallery, and that will keep them from popping up when you open the gallery. However, that won't keep them safe or never viewable.
I would personally suggest not keeping private photos on the phone. If it gets lost, stolen, hacked, spends a few minutes in the hands of a sneaky person, any of these can lead to those photos getting out and being seen by those you don't want seeing them. Save them to a private folder/password protected one on a computer. Much smarter and safer.