Open the Photos app
Select the photo or photos you want to hide
Tap the Share button
Tap Hide
Confirm by tapping Hide Photo
To view hidden photos, go to Albums
Open the Hidden album under Utilities
If needed, enable Hidden Album in Settings
On iPhone or iPad, go to Settings
Tap Photos
Turn on Show Hidden Album
On Mac, open Photos
Select the photo or photos
Control-click or use the Image menu
Choose Hide Photo
Confirm the action