There are several apps in the store to do what you ask. Whether on Android, iOS, or Windows, I simply would not trust them. The security they provide is quite minimal and in many cases, people who know how to use a file explorer to browse are able to still find the photos on the phone.
Many of the apps that actually remove the photos from the phone memory to place them in a secure container will also compress the images and you'll lose a lot of the original picture quality.
My advice is simple. Don't allow pictures on your phone that you don't want others to see. All you need is for someone to walk in on you while you're viewing them and BAM!, you're busted.
My dad is in his mid-70s, and I can guarantee you he is savvy enough to find your pictures hidden with any picture vault on the Android, iOS, or Windows stores. A parent, wife, or girlfriend are seldom as clueless as you take them for.
If you really want such an app, search using multiple keywords like, "photo" "hide" "calculator" etc.