Although this not might be the actual way to hide a pic, we can still use it as its a quick way
1.Open photos app
2.Open camera role
3.Open a pic that you would like to hide
4.Now use the cropping tool to crop a small part of the image
No one will get to know what this is
If you just want to restore it too the original size then just select crop again...
....And bring it back to its original size!
That was quick! Hope now you won't be worrying about someone seeing your pics
The creator of this guide has not included tools