I went over a free Android app that allows you to hide photos and videos on your Android phone :
Hide it pro
How does it work ? Tutorial from the developer :
The moment you launch the app you’ll be presented with a screen similar to the one below. Wtf ?? i did not want an Audio Manager app. What the heck is this. Well this is just a disguise to hide the app itself.
To go to the Actual hide it pro. Long Press on the Audio Manager Icon to access the actual Hide It Pro. btw this Audio Manager is full functional. You can change all the volumes from here directly.
Long press on the title to go to the lock screen. Here you enter you pin/password to login
Then, from next screen you can browse all files in your images/videos gallery, and chose the one you want to move to this secret app.
Warning : The photos are just moved from the public gallery to the locked folder, it means the files can still be viewed when the phone is connected to your computer. But at least, if your SO plays with your phone, she'll just think it's a audio manager app